Show the server map pool in the client map chooser.

Maps that aren't installed are queried from the resource center.

OpenRA.Game/Network/OrderManager.cs

@@ -39,6 +39,9 @@ namespace OpenRA.Network
 		public string ServerError = null;
 		public bool AuthenticationFailed = false;
 
+		// The default null means "no map restriction" while an empty set means "all maps restricted"
+		public HashSet<string> ServerMapPool = null;
+
 		public int NetFrameNumber { get; private set; }
 		public int LocalFrameNumber;
 

OpenRA.Game/Network/UnitOrders.cs

@@ -383,6 +383,12 @@ namespace OpenRA.Network
 						break;
 					}
 
+				case "SyncMapPool":
+					{
+						orderManager.ServerMapPool = FieldLoader.GetValue<HashSet<string>>("SyncMapPool", order.TargetString);
+						break;
+					}
+
 				default:
 					{
 						if (world == null)