run file transfers in threads, not processes

This should fix OSX/BSD issues with not noticing transfer information
files with kqueue. Now that threads are used, the thread can manage the
transfer slot allocation and deallocation by itself; much cleaner.

Assistant/Threads/Transferrer.hs

@@ -74,9 +74,8 @@ runTransfer st dstatus slots t info = case (transferRemote info, associatedFile
 	(Nothing, _) -> noop
 	(_, Nothing) -> noop
 	(Just remote, Just file) -> do
-		tid <- inTransferSlot slots $
-			unsafeForkIOThreadState st $
-				transferprocess remote file
+		tid <- inTransferSlot slots st $
+			transferprocess remote file
 		now <- getCurrentTime
 		runThreadState st $ adjustTransfers dstatus $
 			M.insertWith' const t info