Fixes mask so unmask uses the correct state.

Otherwise the following program would yield False.

    import Control.Distributed.Process
    import Control.Distributed.Process.Node
    import Control.Monad ( void )
    import Network.Transport.TCP

    main :: IO ()
    main = do
      Right transport <- createTransport "127.0.0.1" "8080"
      defaultTCPParameters
      lnid <- newLocalNode transport initRemoteTable
      runProcess lnid $ do
        pid <- getSelfPid
        void $ mask $ \unmask -> do
          spawnLocal $ void $ unmask getSelfPid >>= send pid
          expect >>= liftIO . print . (pid/=)