prefer ipv4 localhost when both are available

The webapp can only run on one of ipv4 and ipv6, no both. Some web browsers
may not support ipv6, so ipv4 is the safe choice.

The actual problem I ran into with it only listening to ipv6 was that
Utility.Url.exists was failing to connect to it. I doubt that haskell's
HTTP library is ipv4 only. More likely, it was only trying one address,
and tried ipv4 first.

Utility/WebApp.hs

@@ -62,15 +62,19 @@ runWebApp app observer = do
 	observer =<< socketPort sock
 
 {- Binds to a local socket, selecting any free port.
+ -
+ - Prefers to bind to the ipv4 address rather than the ipv6 address
+ - of localhost, if it's available.
  -
  - As a (very weak) form of security, only connections from 
  - localhost are accepted. -}
 localSocket :: IO Socket
 localSocket = do
 	addrs <- getAddrInfo (Just hints) (Just localhost) Nothing
-	case addrs of
-		[] -> error "unable to bind to a local socket"
-		(addr:_) -> go addr
+	case (partition (\a -> addrFamily a == AF_INET) addrs) of
+		(v4addr:_, _) -> go v4addr
+		(_, v6addr:_) -> go v6addr
+		_ -> error "unable to bind to a local socket"
 	where
 		hints = defaultHints
 			{ addrFlags = [AI_ADDRCONFIG]