Document how to pass flags to the cli program being debugged.

* core_test: fix TestCoreFpRegisters on go1.9

It was broken by 7bec20e5

* travis-ci: switch to VM builders for linux

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

Depending on how the runtime schedules our goroutines we can get
unlucky and have the first call to runtime.newstack we intercept be for
a different goroutine (usually the garbage collector).
Only check stacktraces that happen on the same goroutine that executed
main.main.

Help out those of us habituated to pdb. <:^)

https://docs.python.org/3/library/pdb.html

* 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 runtime calls into g0 in many places, not necessarily using
runtime.systemstack or runtime.asmcgocall.
One example of this is the call to runtime.newstack inside
runtime.morestack.

If we stop the process while one goroutine is executing
runtime.newstack we would be unable to fully scan its stack because we
don't know that we have to switch back to the goroutine stack after
runtime.newstack.

Instead of tracking down every possible way that the runtime switches
to g0 we switch to the goroutine stack immediately after the top of the
stack, unless cgo is being executed on the systemstack.

Fixes #1066

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

Fixes #1067

Improve the link ordering for the main README and add a
"Getting Started" doc with basic usage information for new users.

I saw a test failure related to this in Travis-CI, if it happens again
I would like to know what's causing it.

Fixes #1052

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.

As of Go 1.8, allows empty GOPATH environment variable.

Sometimes delve on Appveyor can't remove the built binary before
exiting and gets an "Access is denied" error when trying.
See: https://ci.appveyor.com/project/derekparker/delve/build/1527

The last entry of the debug_line table is supposed to be valid for
every PC address greater than its address.

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.

Go 1.10 new method of caching packages means that there is no need to
specify 1.10.

Fixes #186, #83

We need to reset the current file and line number.

Fixes #1008

It can happen if the interface variable escaped, it's out of scope and
we autodereference it.

On macOS, externally linked programs will have an abbrev for
DW_TAG_subprogram without the haschildren flag set. We should handle
this case instead of expecting all DW_TAG_subprogram entries to have
list of children.

Fixes #1034

Prints the frame and frame pointer offsets for each frame.

When creating a stack trace we should switch between the goroutine
stack and the system stack (where cgo code is executed) as appropriate
to reconstruct the logical stacktrace.

Goroutines that are currently executing on the system stack will have
the SystemStack flag set, frames of the goroutine stack will have a
negative FrameOffset (like always) and frames of the system stack will
have a positive FrameOffset (which is actually just the CFA value for
the frame).

Updates #935

Pass CGO_FLAGS='-O0 -g' to go build to disable optimizations when
calling the C compiler.

* Support --output for debug, trace, and test commands

With the `--output` parameter you can configure the output binary. For
example:

    dlv debug --output /tmp/xxx

Will build the binary to `/tmp/xxx`, instead of always putting it as
`debug` in the current directory.

This ensures that the command always works (even if there is already a
file or directory named `debug`) and doesn't write to the source
directory. Especially for things like Delve/Vim integration this is a
good thing to have, I think.

* Address PR feedback and add a test

- We don't need to use `filepath.IsAbs()` on startup; I added that
  because it previously did `"./" + debugname` everywhere, but I don't
  think that's needed at all, since `pathname` without a leading `./`
  implies the current directory.

- Repurpose the existing `TestIssue398` to also test the `--output`
  flag. Also fix an issue where tests wouldn't work if `GOPATH` has
  multiple entries (e..g `GOPATH=$HOME/go:$HOME/mygocode`).

- Print an error if we can't remove the debug binary on exit instead of
  failing silently. Not strictly related to this PR, but a good change
  to add I think.

* Also warn when delve can't remove the binary in test/trace

I only added that to debug, but good to issue this warning consistently.

These are emitted by C compilers but also by the current development
version of the go compiler with the dwarflocationlists flag.