Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

We don't care, at the process level, whether or not we're single
stepping. That state is really only relevant at the thread level.

There's no reason to hang a bunch of commands off of the `info` command.
Promoted all commands to be top level.

Fixes a code path where stacktrace returns < 2 locations and
thread.ReturnAddress would panic. Now returns an error.

This fix helps avoid infinite recursion.

Original implementation was passing the wrong argument.

Breakpoints, tracepoints, etc.. take a location spec as input. This
patch improves the expressiveness of that API. It allows:

* Breakpoint at line
* Breakpoint at function (handling package / receiver smoothing)
* Breakpoint at address
* Breakpoint at file:line
* Setting breakpoint based off regexp

When running `dlv help trace` the help output would display:

dlv trace [regexp] [flags] [flags]

This patch removes the [flags] duplication.

Fixes #188

Passing the '-s' flag to the linker in < Go 1.5 emits an error and
produces a binary that, once codesigned (I believe) will cause an
immediate bus error and terminate.

Fixes #192

Fixes #182

fixes issue #179

For more information see: https://github.com/golang/go/issues/11887