Deadlock is now an exception instead of a return status from
rts_evalIO().

The current behaviour is as follows, and can be changed if necessary:
in the event of a deadlock, the top main thread is taken from the main
thread queue, and if it is blocked on an MVar or an Exception (for
throwTo), then it receives a Deadlock exception.  If it is blocked on
a BLACKHOLE, we instead send it the NonTermination exception.  Note
that only the main thread gets the exception: it is the responsibility
of the main thread to unblock other threads if necessary.

There's a slight difference in the SMP build: *all* the main threads
get an exception, because clearly none of them may make progress
(compared to the non-SMP situation, where all but the top main thread
are usually blocked).