hs_exit()/shutdownHaskell(): wait for outstanding foreign calls to complete before returning
This is pertinent to #1177. When shutting down a DLL, we need to be sure that there are no OS threads that can return to the code that we are about to unload, and previously the threaded RTS was unsafe in this respect. When exiting a standalone program we don't have to be quite so paranoid: all the code will disappear at the same time as any running threads. Happily exiting a program happens via a different path: shutdownHaskellAndExit(). If we're about to exit(), then there's no need to wait for foreign calls to complete.