Add Engine.DefaultMod argument to override modchooser fallback.

OpenRA.Game/Game.cs

@@ -368,8 +368,8 @@ namespace OpenRA
 			ModData = null;
 
 			// Fall back to default if the mod doesn't exist or has missing prerequisites.
-			if (!IsModInstalled(mod))
-				mod = new GameSettings().Mod;
+			if (mod == null || !IsModInstalled(mod))
+				mod = args.GetValue("Engine.DefaultMod", "modchooser");
 
 			Console.WriteLine("Loading mod: {0}", mod);
 			Settings.Game.Mod = mod;

OpenRA.Game/Settings.cs

@@ -153,8 +153,8 @@ namespace OpenRA
 
 	public class GameSettings
 	{
-		[Desc("Load a specific mod on startup. Shipped ones include: ra, cnc and d2k")]
-		public string Mod = "modchooser";
+		[Desc("Load a specific mod on startup.")]
+		public string Mod = null;
 		public string PreviousMod = "ra";
 
 		public string Platform = "Default";