When handling external call-ins (via the RTS API) in
the multi-threaded case, add the StgMainThread that
the external thread is going to block waiting on
to the main_threads list prior to scheduling the new
worker thread.

Do this by having the scheduler provide a new entry
point, scheduleWaitThread().

Fixes a bug/race condition spotted by Wolfgang Thaller
(see scheduleWaitThread() comment) + enables a little
tidier interface between RtsAPI and Schedule.