llgs: more work on thread stepping.

See https://github.com/tfiala/lldb/issues/75.  Not fixed yet but
continuing to push this further.

Fixes:
* Resume() now skips doing deferred notifications if we're doing a
  vCont;{c,C}.  In this case, we're trying to start something up,
  not defer a stop notification.  The default thread action stop
  mode pickup was triggering a stop because it had at least one
  stop, which was wrong in the case of a continue.  (Bug introduced
  by previous change.)

* Added a variant to ThreadStateCoordinator to specify a set of
  thread ids to be skipped when triggering stop notifications to
  non-stopped threads on a deferred signal call.  For the case of
  a stepping thread, it is actually told to step (and is running)
  for a brief moment, but the thread state coordinator would think
  it needed to send the stepping thread a stop, which id doesn't
  need to do.  This facility allows me to get around that cleanly.

With this change, behavior is now reduced to something I think is
essentially a different bug:

* Doing a step into libc code from my code crashes llgs.
* Doing a next out of a function in my own code crashes llgs.