Fix #4514 - IO manager deadlock
* The public APIs for threadWaitRead and threadWaitWrite remain unchanged,
and now throw an IOError if a file descriptor is closed behind their
backs. This behaviour is documented.
* The GHC.Conc API is extended to add a closeFd function, the behaviour
of which is documented.
* Behind the scenes, we add a new evtClose event, which is used only when
one thread closes a file descriptor that other threads are blocking on.
* Both base's IO code and network use the new closeFd function.