port transferkeys to windows; make stopping in progress transfers work too (probably)

transferkeys had used special FDs for communication, but that would be
quite annoying to do in Windows.

Instead, use stdin and stdout. But, to avoid commands like rsync stomping
on them and messing up the communications channel, they're duplicated to a
different handle; stdin is replaced with a null handle, and stdout is
replaced with a copy of stderr. This should all work in windows too.

Stopping in progress transfers may work on windows.. if the types unify
anyway. ;) May need some more porting.

Assistant/TransferSlots.hs

@@ -38,6 +38,8 @@ import qualified Control.Concurrent.MSemN as MSemN
 #ifndef mingw32_HOST_OS
 import System.Posix.Process (getProcessGroupIDOf)
 import System.Posix.Signals (signalProcessGroup, sigTERM, sigKILL)
+#else
+import System.Win32.Console (generateConsoleCtrlEvent, cTRL_C_EVENT, cTRL_BREAK_EVENT)
 #endif
 
 type TransferGenerator = Assistant (Maybe (Transfer, TransferInfo, Transferrer -> Assistant ()))
@@ -252,18 +254,21 @@ cancelTransfer pause t = do
 	signalthread tid
 		| pause = throwTo tid PauseTransfer
 		| otherwise = killThread tid
+	{- In order to stop helper processes like rsync,
+	 - kill the whole process group of the process
+	 - running the transfer. -}
 	killproc pid = void $ tryIO $ do
 #ifndef mingw32_HOST_OS
-		{- In order to stop helper processes like rsync,
-		 - kill the whole process group of the process
-		 - running the transfer. -}
 		g <- getProcessGroupIDOf pid
 		void $ tryIO $ signalProcessGroup sigTERM g
-		threadDelay 50000 -- 0.05 second grace period
+		graceperiod
 		void $ tryIO $ signalProcessGroup sigKILL g
 #else
-		error "TODO: cancelTransfer not implemented on Windows"
+		void $ tryIO $ generateConsoleCtrlEvent cTRL_C_EVENT pid
+		graceperiod
+		void $ tryIO $ generateConsoleCtrlEvent cTRL_BREAK_EVENT pid
 #endif
+	graceperiod = threadDelay 50000 -- 0.05 second
 
 {- Start or resume a transfer. -}
 startTransfer :: Transfer -> Assistant ()