Add a 'Shutting down' state to the gameserver.

Tell the masterserver about it, so it can quickly remove games
from the list when they are finished, rather than waiting for the
5 minute TTL to expire.

OpenRA.Game/Game.cs

@@ -313,7 +313,8 @@ namespace OpenRA
 					{
 						System.Threading.Thread.Sleep(100);
 
-						if((server.GameStarted)&&(server.conns.Count<=1))
+						if((server.State == Server.ServerState.GameStarted)
+						    && (server.conns.Count<=1))
 						{
 							Console.WriteLine("No one is playing, shutting down...");
 							server.Shutdown();