Add CVar for changing room ID

A pet peeve of mine is that you cannot change the room ID on the fly on a dedicated server without restarting. This patch makes this possible by adding a CVar named masterserver_room_id, which allows changing the room ID at any point - even after the server has started. It also allows setting the room ID via a configuration file, so passing -room is no longer necessary (of course, it's still available for backwards compatibility reasons, and I don't see a point of removing it, either).

NOTE: The unlisted room ID has been changed from -1 to 0, but the effect is still the same since the logic that checks if it should register on the MS is ms_RoomID > 0.