Threaded RTS:
Don't start new worker threads earlier than necessary.
After this commit, a Haskell program that uses neither forkOS nor forkIO is
really single-threaded (rather than using two OS threads internally).

Some details:
Worker threads are now only created when a capability is released, and
only when
(there are no worker threads)
	&& (there are runnable Haskell threads ||
	    there are Haskell threads blocked on IO or threadDelay)
awaitEvent can now be called from bound thread scheduling loops
(so that we don't have to create a worker thread just to run awaitEvent)