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/Pairing/Network.hs

@@ -60,7 +60,7 @@ multicastPairMsg repeats secret pairdata stage = go M.empty repeats
 		go cache' $ pred <$> n
 	{- The multicast library currently chokes on ipv6 addresses. -}
 	sendinterface _ (IPv6Addr _) = noop
-	sendinterface cache i = void $ catchMaybeIO $
+	sendinterface cache i = void $ tryIO $
 		withSocketsDo $ bracket setup cleanup use
 	  where
 		setup = multicastSender (multicastAddress i) pairingPort