* 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.