async exception safety for openFd

Audited for openFile and openFd, and this fixes all the ones I found
where an async exception could prevent the file getting closed.

Except for the lock pool, which is a whole other can of worms.

Annex/Init.hs

@@ -230,7 +230,7 @@ probeLockSupport = withEventuallyCleanedOtherTmp $ \tmp -> do
   where
 	go f mode = do
 		nukeFile f
-		let locktest = 
+		let locktest = mask $ const $
 			Posix.lockExclusive (Just mode) f
 				>>= Posix.dropLock
 		ok <- isRight <$> tryNonAsync locktest