Fix #1185: restart the IO manager after fork()

Fix #1185: restart the IO manager after fork()

This is the libraries/base part of the patch; there is a corresponding
patch to GHC itself.

The main change is that we now keep track of the IO manager's ThreadId
in a top-level MVar, and ensureIOManagerIsRunning checks whether a
previous IO manager thread is alive before starting one.  In the child
of fork(), we can hence call ensureIOManagerIsRunning to restart the
IO manager.