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

Process is an incorrect name for the DebuggedProcess struct that the
thread is "a part" of. Also, no need to export that field.

This information is hardly going to be useful to a user of Delve and if
the user really needs information on new threads they can simply using
the provided commands to list and switch between threads.

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

* Better tracking of current goroutine
* More efficient, eliminates superfluous step syscalls
* Handles concurrency and thread coordination better