First of a few refactors to allow multiple clients / frontends for
Delve. Current implementation now uses a cli client, but conceivably we
could have an http or socket based client as well.

Since fixing scheduler handling bugs, a new bug was exposed where Step
was calling Clear off of the DebuggedProcess struct. This is incorrect,
 and should be handled by the thread itself and not delegated.

This remove reliance on the procfs for figuring out what threads are
already active when we attach to a running process. The allm linked list
will be present to matter what OS we're on, whereas procfs will not be
present everywhere.

This is the first in a series of steps to support more platforms.

Remove any assumption that a wait syscall on a thread id after a
continue will return. Any time we continue a thread, wait for activity
from any thread, because the scheduler may well have switched contexts
on us due to syscall entrace, channel op, etc...

There are several more things to be done here including:

* Potential tracking of goroutine id as we jump around to thread
  contexts.
* Potential of selectively choosing threads to operate on based on the
  internal M data structures, ensuring that our M has an active G.

This commit partially fixes #23 and #24, however there are still some
random hangs that happen and need to be ironed out.

Remove reliance on order of dwarf instructions.