fixed the multicast socket fd leak

Turns out sClose was working fine.. but it was not being run on every
opened socket. The upstream bug is that multicastSender can crash
on an invalid (or ipv6) address and when this happens it's already
opened a socket, which just goes missing with no way to close it.
A simple fix to the library can avoid this, as I describe here:
https://github.com/audreyt/network-multicast/issues/2

In the meantime, just skipping ipv6 addresses will fix the fd leak.

Assistant/Pairing/Network.hs

@@ -57,6 +57,8 @@ multicastPairMsg repeats secret pairdata stage = go M.empty repeats
 			mapM_ (sendinterface cache') addrs
 			threadDelaySeconds (Seconds 2)
 			go cache' $ pred <$> n
+		{- The multicast library currently chokes on ipv6 addresses. -}
+		sendinterface cache (IPv6Addr _) = noop
 		sendinterface cache i = void $ catchMaybeIO $
 			withSocketsDo $ bracket setup cleanup use
 			where