Minidumps are the windows equivalent of unix core files.
This commit updates pkg/proc/core so that it can open and read windows
minidumps.

Updates #794

Move windows register handling code to winutil package in preparation
for using it in proc/core to read minidump files.

If a function can be inlined it will appear as two entries in
debug_info. A DW_TAG_subprogram entry with DW_AT_inlined = true (that
will be used as the abstract origin) and a second DW_TAG_subprogram
entry with an abstract origin.
To retrieve the name of this second entry we must load its abstract
origin.

If proc.Step encounters a CALL instruction that points to an address
that isn't associated with any function it should still follow the
CALL.

The circumstances creating this problem do not normally occur, it was
encountered in the process of fixing a bug created by Go1.12.

It was never true that return variables were in the inverse order.
Instead in Go1.11 return variables are saved in debug_info in an
arbitrary order and inverting them just happened to work for this
specific example.

This bug was fixed in Go 1.12, regardless we should attempt to
rearrange return variables anyway.

When a location expression requests a register check that we have as
many bytes in the register as requested and if we don't report the
error.

Updates #1416

Instead of unconditionally returning all present goroutines,
GoroutinesInfo now allows specifying a range (start and count). In
addition to the array of goroutines and the error, it now also returns
the next goroutine to be processed, to be used as 'start' argument on
the next call, or 0 if all present goroutines have already been
processed.

This way clients can avoid eating large amounts of RAM while debugging
core dumps and processes with a exceptionally high amount of goroutines.

Fixes #1403

This patch is a slight refactor to share more code used for genericprocess initialization. There will always be OS/backend specificinitialization, but as much as can be shared should be to preventduplicating of any logic (setting internal breakpoints, loading bininfo,etc).

Users can create sparse maps in two ways, either by:
a) adding lots of entries to a map and then deleting most of them, or
b) using the make(mapType, N) expression with a very large N

When this happens reading the resulting map will be very slow
because loadMap needs to scan many buckets for each entry it finds.

Technically this is not a bug, the user just created a map that's
very sparse and therefore very slow to read. However it's very
annoying to have the debugger hang for several seconds when trying
to read the local variables just because one of them (which you
might not even be interested into) happens to be a very sparse map.

There is an easy mitigation to this problem: not reading any
additional buckets once we know that we have already read all
entries of the map, or as many entries as we need to fulfill the
MaxArrayValues parameter.

Unfortunately this is mostly useless, a VLSM (Very Large Sparse Map)
with a single entry will still be slow to access, because the single
entry in the map could easily end up in the last bucket.

The obvious solution to this problem is to set a limit to the
number of buckets we read when loading a map. However there is no
good way to set this limit.
If we hardcode it there will be no way to print maps that are beyond
whatever limit we pick.
We could let users (or clients) specify it but the meaning of such
knob would be arcane and they would have no way of picking a good
value (because there is no objectively good value for it).

The solution used in this commit is to set an arbirtray limit on
the number of buckets we read but only when loadMap is invoked
through API calls ListLocalVars and ListFunctionArgs. In this way
`ListLocalVars` and `ListFunctionArgs` (which are often invoked
automatically by GUI clients) remain fast even in presence of a
VLSM, but the contents of the VLSM can still be inspected using
`EvalVariable`.

Adds a config file option to allow specifying a list of directories to
search in when looking for seperate external debug info files.

Fixes #1353

Make code in core.go OS and architecture agnostic in preparation for
adding Windows minidump support.

Some tests used a fake vendor directory placed inside _fixtures to
import some support packages.
In go.mod mode vendor directory are only supported on the root of the
project, which breaks some of our tests.
Since vendor directories outside the root of the project are so rare
anyway it's possible that a future version of go will stop supporting
it even in GOPATH mode.
Also it was weird and unnecessary in the first place anyawy.

For some reason the Go file pkg/dwarf/reader/reader.go had the exec bit
set on it. This patch removes that.

The linux version of proc/native and proc/core contained largely
overlapping implementations of the register handling code, deduplicate
it by moving it into proc/linutil.

This patch allows the `trace` CLI subcommand to display return values of
a function. Additionally, it will also display information on where the
function exited, which could also be helpful in determining the path
taken during function execution.

Fixes #388

Some libraries (for example steam_api64.dll) will send this exception
code to set the thread name on Microsoft VisualC.
In theory it should be fine to send the exception back to the target,
which is responsible for setting a handler for it, in practice in some
cases (steam_api64.dll) this will crash the program. So we'll mask it
instead.

Fixes #1383

Fixes #1378

Adds an --exec flag for the trace subcommand allowing users to specify a
pre-compiled binary to exec and trace.

Also adds a --test flag as a convienance for compiling and tracing a
test binary.

Fixes #1073

Fix a bug where the regexp would be used as a package string
incorrectly. This would manifest when trying to execute the trace
subcommand as `dlv trace <regexp>` without specifying a package
identifier. The `<regexp>` would be passed to the Go build command
causing an error.

ContinueOnce didn't detect the way RR signals that it has reached the
start of the process and would never finish.

Fixes #1376

Continue did not resume execution after a call to CallFunction if the
point where the process was stopped, before the call CallFunction, was
a breakpoint.

Fixes #1374

The name "where" may confuse users into thinking that this parameter
actually does something where in fact it's just arbitrary text used to
identify the checkpoint.

Fixes #1373

Add ability to evaluate variables on the scope of a deferred call's
argument frame.

Fix escape checking in function calls  and add a flag to disable it.

Extend `string()`
1.convert `byte/rune array`(not only `slice`) to string.
2.convert string to string(itself), just like `string(str)` in go language.

Support for position independent executables (PIE) on the native linux
backend, the gdbserver backend on linux and the core backend.
Also implemented in the windows native backend, but it can't be tested
because go doesn't support PIE on windows yet.

This change adds a config flag to specify the foreground color of
line numbers for the source list command.

Use the defination of Server interface in service package, instead of
temporary in func.

Replace the socket connection with an in-memory connection (created by net.Pipe) for non-headless uses of delve.
This is faster and more secure.

Fixes #1332

On macOS 10.14 Apple changed the command line tools so that system
headers now need to be manually installed.

Instead of adding one extra install step to the install procedure add a
build tag to allow compilation of delve without the native backend on
macOS. By default (i.e. when using `go get`) this is how delve will be
compiled on macOS, the make script is changed to enable compiling the
native backend if the required dependencies have been installed.

Insure that both configuration still build correctly on Travis CI and
change the documentation to describe how to compile the native backend
and that it isn't normally needed.

Fixes #1359

With this syntax users do not need to type the concrete type of an
interface variable to access its contents. This also sidesteps the
problem where the serialization of a type by go/printer is different
from the one used for debug_info type names.

Updates #1328

Use IsInternal and IsUser of Breakpoint, not duplicate code

We should print something when we exit from continue/next/step/stepout
even if we don't have a source file for the instruction that we are
stopped on.

This is mostly important on macOS where a SIGSEGV will cause 'continue'
to fail with a 'bad access' error (see #852) and the output can be
confusing.

Fixes #1244