Common mods support

Implement "common mods", includeable from
  {$user,$share}/games/common/$modname
by using the game.conf setting
  common_mods = $modname,$modname2,...

src/subgame.h

@@ -24,6 +24,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <set>
 #include <vector>
 
+class Settings;
+
 #define WORLDNAME_BLACKLISTED_CHARS "/\\"
 
 struct SubgameSpec
@@ -52,6 +54,8 @@ struct SubgameSpec
 	}
 };
 
+bool getGameConfig(const std::string &game_path, Settings &conf);
+
 std::string getGameName(const std::string &game_path);
 
 SubgameSpec findSubgame(const std::string &id);