[mingw only]
Better handling of I/O request abortions upon throwing an exception
to a Haskell thread. As was, a thread blocked on an I/O request was
simply unblocked, but its corresponding worker thread wasn't notified
that the request had been abandoned.

This manifested itself in GHCi upon Ctrl-C being hit at the prompt -- the
worker thread blocked waiting for input on stdin prior to Ctrl-C would
stick around even though its corresponding Haskell thread had been
thrown an Interrupted exception. The upshot was that the worker would
consume the next character typed in after Ctrl-C, but then just dropping
it. Dealing with this turned out to be even more interesting due to
Win32 aborting any console reads when Ctrl-C/Break events are delivered.

The story could be improved upon (at the cost of portability) by making
the Scheduler able to abort worker thread system calls; as is, requests
are cooperatively abandoned. Maybe later.

Also included are other minor tidyups to Ctrl-C handling under mingw.

Merge to STABLE.