Add support for the IO manager thread on Windows

Add support for the IO manager thread on Windows
Fixes #637.
The implications of this change are:
  - threadDelay on Windows no longer creates a new OS thread each time,
    instead it communicates with the IO manager thread in the same way as
    on Unix.
  - deadlock detection now works the same way on Windows as on Unix; that
    is the timer interrupt wakes up the IO manager thread, which causes
    the scheduler to check for deadlock.
  - Console events now get sent to the IO manager thread, in the same way as
    signals do on Unix.  This means that console events should behave more
    reliably with -threaded on Windows.

All this applies only with -threaded.  Without -threaded, the old
ConsoleEvent code is still used.

After some testing, this could be pushed to the 6.6 branch.