Added mechanism to stop threads and restart them. When a

Added mechanism to stop threads and restart them.  When a
derived-thread error is signalled, the signalling thread stops itself
to allow debugging to occur.  The debugger recognizes such stopped
threads and restarts them appropriately when the user requests a
continuation.

Also changed the handling of the thread timer to vary the period of
the timer in cases where the next desired timer event is longer than
the default timer interval.