- move forkIO into GHC.Conc, so that the I/O manager can use proper forkIO
  with an exception handler.  This required TopHandler.lhs-boot.  It's the
  right thing, though, since the forkIO implementation is GHC-specific.

- check for out-of-range file descriptors in the I/O manager, rather than
  just exploding.  The I/O manager will exit ungracefully, but at least
  there will be an error message.