FIX #1177, partially at least.

Now we don't wait for outstanding IO requests when shutting down at
program exit time, but we still wait when shutting down a DLL (via
hs_exit()).  There ought to be a better way to do this, but
terminating the threads forcibly is not a good idea (it never is: the
thread might be holding a mutex when it dies, for example).

I plan to add some docs to the user guide to describe how to shut
down a DLL properly.