llgs: fix up some handling of stepping.

Tracked down while working on https://github.com/tfiala/lldb/issues/75.
This is not a complete fix for that issue, but moves us farther along.

Fixes:
* When a thread step is requested via vCont:{s,S}, Resume() now marks
  the stepping thread as (1) currently stepping and (2) does trigger
  the deferred signal for the stepped thread.  This fixes a bug where
  we were actually triggering a deferred stop cycle here for the non-stepping
  thread since the single step thread was not part of the Resume()
  deferred signal mechanism.  The stepping thread is also marked in
  the thread state coordinator as running (via a resume callback).

* When we get the SIGTRAP signal for the step completion, we don't
  do a deferred signal call - that happened during the vCont:{s,S}
  processing in Resume() already.  Now we just need to mark that
  the stepping thread is now stopped.  If this is the last thread
  in the set that needs to stop, it will trigger the process/delegate
  stop call that will notify lldb.  Otherwise, that'll happen when
  the final thead we're waiting for stops.

Misc:
* Fixed up thread stop logging to use a leading 0 (0x%PRIx32) so
  we don't get log lines like 0x5 for 0x05 SIGTRAP.