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/=)