crippled filesystem support, probing and initial support

git annex init probes for crippled filesystems, and sets direct mode, as
well as `annex.crippledfilesystem`.

Avoid manipulating permissions of files on crippled filesystems.
That would likely cause an exception to be thrown.

Very basic support in Command.Add for cripped filesystems; avoids the lock
down entirely since doing it needs both permissions and hard links.
Will make this better soon.

Assistant/WebApp/Utility.hs

@@ -97,7 +97,7 @@ cancelTransfer pause t = do
 		| 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 $ catchMaybeIO $ do
+	killproc pid = void $ tryIO $ do
 		g <- getProcessGroupIDOf pid
 		void $ tryIO $ signalProcessGroup sigTERM g
 		threadDelay 50000 -- 0.05 second grace period