Partial fix for #926
It seems that when a program exits with open DLLs on Windows, the system attempts to shut down the DLLs, but it also terminates (some of?) the running threads. The RTS isn't prepared for threads to die unexpectedly, so it sits around waiting for its workers to finish. This bites in two places: ShutdownIOManager() in the the unthreaded RTS, and shutdownCapability() in the threaded RTS. So far I've modified the latter to notice when worker threads have died unexpectedly and continue shutting down. It seems a bit trickier to fix the unthreaded RTS, so for now the workaround for #926 is to use the threaded RTS.