Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.

Hardware breakpoints are by definition architecture dependant. Move them
off the DebuggedProcess struct and onto the associated arch struct.

* Set hardware breakpoints on all existing threads
* Set hardware breakpoints on any new thread the spawns

Fixes #111

* Remove half baked CallFn implementation.
* Cleanup whitespace
* Cleanup read/write mem functions

Prevents leaving around a bunch of zombie processes

* Ignore signals we do not care about
* Implement custom fork/exec for Darwin to convert signals to mach
  exceptions

* Properly find next source line for goroutines blocked in chanrecv
* Refactor breakpoint clearing
* Refactor temp breakpoint setting

* DebuggedProcess method: `PrintGoroutinesInfo` -> `GoroutinesInfo`
* `goroutines` command in command/command.go now responsible for
  printing info.