When the vars, locals, or args commands return no results, nothing is

printed out to the terminal. This commit makes these commands print a

message like `(no locals)` when there is nothing to show. This feedback

is more descriptive of what is being returned than an empty string.

I ran through these instructions and didn't notice that the
certificate I'd created appeared three times in the list I
was looking at.  Emphasize where the trust setting is
enabled.

proc.(*Thread).Scope fails if we can find a FDE but PCToLine returns
nothing

- made GoroutineStacktrace a method of struct G
- made stacktrace a method of StackIterator
- renamed StackIterator to stackIterator
- factored out logic to obtain a stackIterator from a goroutine that's
used by both (*G).Stacktrace and by (*G).UserCurrent

Instead of returning an error when FDE of a frame can not be found,
just truncate the stack trace.

Fixes #462

It is sometimes impossible to convert an EvalScope immediately after
starting the program but disassembly can still proceed.

Fixes #403

On linux kernel 2.6.x, the Trace/Stop status is 'T'

Fixes #209

It was unclear that to install, first clone Delve before `CERT=mycert make install`
Added link to helpful walkthrough installation video

Automatically sets a breakpoint on runtime.startpanic, the function
that gets called by runtime.dopanic when a panic is not recovered.

Implements #317

Caches the mapping of type names to offset in debug_info to speed up
variable evaluation.

BEFORE:
	BenchmarkArray-4         	     100	  13'238'441 ns/op	   0.62 MB/s
	BenchmarkArrayPointer-4  	     200	  10'044'093 ns/op	   0.87 MB/s
	BenchmarkMap-4           	    1000	   1'332'530 ns/op	   0.77 MB/s
	BenchmarkGoroutinesInfo-4	      10	 114'677'462 ns/op
	BenchmarkLocalVariables-4	    2000	   1'223'975 ns/op
AFTER:
	BenchmarkArray-4         	     200	   9'925'686 ns/op	   0.83 MB/s
	BenchmarkArrayPointer-4  	     100	  11'143'930 ns/op	   0.78 MB/s
	BenchmarkMap-4           	    2000	   1'302'520 ns/op	   0.79 MB/s
	BenchmarkGoroutinesInfo-4	      30	  35'079'549 ns/op
	BenchmarkLocalVariables-4	    1000	   1'137'299 ns/op

Note in particular the speedup of BenchmarkGoroutinesInfo, since
proc.(*Variable).parseG is a function we call a lot.

The go compiler changed and break statements no longer get compiled to
nothing when -N is passed:

https://go-review.googlesource.com/#/c/19848/
https://go-review.googlesource.com/#/c/19854/

BreakpointInfo variables were always evaluated on CurrentThread
instead of the thread that triggered the breakpoint.

Implements extensible mechanism to specify which commands accept
prefixes (goroutine, frame, on) instead of hardcoding them in
a switch.

Implements #240

Lists all the types defined in the debugged program.

Fixes #414

Fixes #411

proc.(*Process) methods are not thread safe, multiple clients
connecting simultaneously to a delve server (Issue #383) or a even
a single over-eager client (Issue #408) can easily crash it.
Additionally (Issue #419) calls to client.(*RPCClient).Halt can
crash the server because they can result in calling the function
debug/dwarf.(*Data).Type simultaneously in multiple threads which
will cause it to return incompletely parsed dwarf.Type values.

Fixes #408, #419 (partial)

Fixes #419 (partial)

Going forward, all documentation should be placed in the Documentation
directory in the root of the project. This switch allows maintainers to
approve updates to documentation before they are committed, as opposed
to the pre-existing wiki which anybody could modify.

Currently the Documentation directory includes docs on building, usage,
and minimal docs around the API. This is just the initial commit, and
documentation will continue to improve over time.

Some changes have been made (and will continue to be made) to `cmd/dlv` to
ensure we can auto-generate documentation for all commands from the
newly provided script `scripts/gen-usage-docs.go`, which can be invoked
via `go run scripts/gen-usage-docs.go`.

Additionally, version has been split into its own package. This was a
bit of housekeeping related to the changes made the `cmd/dlv`.

Unrelated to conversion, I have also changed (*Thread).readMemory
to return only first count bytes of memory just as advised
by ReadProcessMemory.

Fixes #409
Fixes #412
Fixes #416