use Mono.Nat for UPnP port forwarding

- might support more devices
- supports internal and external port mapping
- discover the device only once (at startup)

OpenRA.Game/GameRules/Settings.cs

@@ -29,7 +29,7 @@ namespace OpenRA.GameRules
 		public int ExternalPort = 1234;
 		public bool AdvertiseOnline = true;
 		public string MasterServer = "http://master.open-ra.org/";
-		public bool AllowUPnP = false;
+		public bool AllowUPnP = true;
 		public bool AllowCheats = false;
 		public string Map = null;
 		public string[] Ban = null;