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.

Wrap syscall.Wait4 and cleanup a few coordination issues.

There are still some issues here where background threads are left
sleeping. This could potentially cause weird issues. There are a few
more things I have planned to cleanup thread coordination issues.

* Remove hardcoded string and use command help message
* Include aliases in output

Added:
- break: b
- continue: c
- step: si
- next: n
- print: p

Taken from the gdb command list

Also updated .gitignore to ignore built dlv file in cmd/dlv

Properly scope variable evaluation to the function the traced process is
currently stopped in.