New implementation & changed type signature of forkProcess

forkProcess now has the following type:
forkProcess :: IO () -> IO ProcessID

forkProcessAll has been removed as it is unimplementable in the threaded RTS.

forkProcess using the old type (IO (Maybe ProcessID)) was impossible to
implement correctly in the non-threaded RTS and very hard to implement
in the threaded RTS.
The new type signature allows a clean and simple implementation.