Refactors some code, adds a bunch of docstrings and just generally fixes
a bunch of linter complaints.

Add a flag to Stackframe that indicates where the stack frame is the
bottom-most frame of the stack. This allows clients to know whether the
stack trace terminated normally or if it was truncated because the
maximum depth was reached.
Add a truncation message to the 'stack' command.

Normally variables that have a named struct as a type will get a
typedef entry as their type, sometimes however the Go linker will
decide to use the DW_TAG_structure_type entry instead.

For consistency always wrap a struct type into a typedef when we are
creating a new variables (see comment in newVariable for exceptions).

This fixes a bug where it would be impossible to call methods on a
global variable.

Explain why we are asking the user to write to a /proc file.

Evaluates var.method expressions into a variable holding the
corresponding method with the receiver variable as a child, in
preparation for extending CallFunction so that it can call methods.

Changes (*Variable).setValue so that it can be used in CallFunction to
set up the argument frame for the function call, adding the ability to:
- nil nillable types
- set strings to the empty string
- copy from one structure to another (including strings and slices)
- convert any interface type to interface{}
- convert pointer shaped types (map, chan, pointers, and structs
  consisting of a single pointer field) to interface{}

This covers all cases where an assignment statement can be evaluated
without allocating memory or calling functions in the target process.

Adds -defer flag to the stack command that decorates the stack traces
by associating each stack frame with its deferred calls.

Reworks proc.next to use this feature instead of using proc.DeferPC,
laying the groundwork to implement #1240.

Implements the function call injection protocol introduced in go 1.11
by https://go-review.googlesource.com/c/go/+/109699.

This is only the basic support, see TODO comments in pkg/proc/fncall.go
for a list of missing features.

Updates #119

Output a better error message when users try to debug a Go 1.11
executable but delve was not built with Go 1.11.

Adds StartPC to proc.G, StartLoc to api.Goroutine and show the start
location in the command line client when appropriate.

Updates #1104

Setting the Level field of a logrus logger doesn't actually do anything
since the Level field simply reports the log level of the last log
message emitted on the logger.
The right way to do that is to set logger.Logger.Level.
Also cleans up newline characters from log messages emitted through
logrus and fixes the direction of the arrows in the messages emitted by
rpccommon, which was inconsistent with the arrows of gdbserial.

Adds a field, DeclLine, to Variable containing the declaration line
number of the variable.

This pull request makes several changes to delve to allow headless
instancess that are started with the --accept-multiclient flag to
keep running even if there is no connected client. Specifically:

1. Makes a headless instance started with --accept-multiclient quit
    after one of the clients sends a Detach request (previously they
    would never ever quit, which was a bug).
2. Changes proc/gdbserial and proc/native so that they mark the
    Process as exited after they detach, even if they did not kill the
    process during detach. This prevents bugs such as #1231 where we
    attempt to manipulate a target process after we detached from it.
3. On non --accept-multiclient instances do not kill the target
    process unless we started it or the client specifically requests
    it (previously if the client did not Detach before closing the
    connection we would kill the target process unconditionally)
4. Add a -c option to the quit command that detaches from the
    headless server after restarting the target.
5. Change terminal so that, when attached to --accept-multiclient,
    pressing ^C will prompt the user to either disconnect from the
    server or pause the target process. Also extend the exit prompt to
    ask if the user wants to keep the headless server running.

Implements #245, #952, #1159, #1231

This message is used by clients to determine the port that a headless
instance is using, therefore the format can not change or move to a
different file handle.

Fixes #1245

Implements structured logging via Logrus. This gives us a logger per
boundry that we care about, allowing for easier parsing of logs if users
have more than one log option enabled. Also, cleans up a lot of
conditionals in the code by simply silencing the logger at creation as
opposed to conditionally logging everywhere.

If the application being debugged imports two packages with the same
name (but different paths) there was no way to disambiguate the two,
since the character '/' can not appear inside a go identifier.

By allowing users to use a string literal as the package name a package
path can be specified.

We occasionally receive bug reports from users of VSCode-go and GoLand.
GoLand has its own way of capturing the packet exchange between itself
and delve but VSCode-go (supposedly) doesn't.
So far this hasn't been a problem since all bug reports were obvious
bugs on the plugin or easy to reproduce without VSCode-go, but it might
be helpful in the future to have a way to log the packet exchange
between dlv and a frontend.

This commit adds a --log-output option to enable logging of all rpc
messages and changes service/rpccommon accordingly.

Displays the return values of the current function when we step out of
it after executing a step, next or stepout command.

Implementation of this feature is tricky: when the function has
returned the return variables are not in scope anymore. Implementing
this feature requires evaluating variables that are out of scope, using
a stack frame that doesn't exist anymore.

We can't calculate the address of these variables when the
next/step/stepout command is initiated either, because between that
point and the time where the stepout breakpoint is actually hit the
goroutine stack could grow and be moved to a different memory address.

Change the linux verison of proc/native and proc/gdbserial (with
debugserver) so that they let the target process use the terminal when
delve is launched in headless mode.

Windows already worked, proc/gdbserial (with rr) already worked.
I couldn't find a way to make proc/gdbserial (with lldb-server) work.

No tests are added because I can't think of a way to test for
foregroundness of a process.

Fixes #65

Fixes #951

Caching the frame in variablesByTag is problematic:

1. accounting for variables that are (partially) stored in registers is
complicated (see issue #1106)
2. for some types (strings, interfaces...) simply creating the Variable
object reads memory, which therefore happens before we can do any
caching.

Instead cache the entire frame when the EvalScope object is created.
The cached range is between the SP value of the current frame and the
CFA of the preceeding frame, if available, or the CFA of the current
frame otherwise.

Fixes #1106

Change evaluation of binary operators so that both && and || only
evaluate their second argument conditionally, like go does.

When gdbserial can not find debugserver or lldb-server the error
message is always the same and it complains about lldb-server not being
found.

This is fine on linux (where the backend is unnecessary) but incomplete
on macOS (where the backend is actually used).

Make the error message clearer so that users who do not bother reading
install instructions are not confused.

updates vendored version of x86asm, adds a symbol lookup function to
pass to the disassembler.

This will show global symbol names in the disassembly like go tool
objdump does.

Fixes #1151

The proper way to stop a running process is to call RequestManualStop,
which most code already did with the exception of some old test code.

debug_info entries can use DW_AT_abstract_origin to inherit the
attributes of another entry, supporting this attribute is necessary to
support DW_TAG_inlined_subroutine.

Go, starting with 1.10, emits DW_TAG_inlined_subroutine entries when
inlining is enabled.

Much like the bug in issue #1031 and commit
f6f6f0bf pointers can also escape to
the heap and then have a zero address (and no children) when we
autodereference.

1. Mark autodereferenced escaped variables with a 0 address as
   unreadable.
2. Add guards to the pretty printers for unsafe.Pointer and pointers.

Fixes #1075

* command/terminal: allow restart to change process args

Add -args flag to "restart" command. For example, "restart -args a b c" will
pass args a b c to the new process.

Add "-c" flag to pass the checkpoint name. This is needed to disambiguate the
checkpoint name and arglist.

Reverted unnecessary changes.

* Applied reviewer comments.

Vendored argv.

Change the syntax of restart. When the target is is in recording mode, it always
interprets the args as a checkpoint. Otherwise, it interprets the args as
commandline args. The flag "-args" is still there, to handle the case in which
the user wants to pass an empty args on restart.

* Add restartargs.go.

Change "restart -args" to "restart -noargs" to clarify that this flag is used to
start a process with an empty arg.

The rr backend doesn't report the exit status (the argument of the W
packet seems to always be 0).

Fixes #1067

Adds a configuration option (show-location-expr) that when activated
will cause the whatis command to also print the DWARF location
expression for a variable.

So far we have evaluated the locspec "+0" the same way we evaluate all
"+n" locspecs, this means that we turn the current PC into a file:line
pair, then we turn back the file:line into a PC address.

Normally this is harmless, however all autogenerated code returns the
source position "<autogenerated>:1" which resolves back to the very
first autogenerated instruction in the code.

This messes up the behaviour of the "disassemble" command which uses
the locspec "+0" to figure out what code to disassemble if no arguments
are passed.

We should make +0 always resolve to the current PC (of the given scope)
so that clients can use +0 as a default locspec.