Set mapgen_singlenode to a solid node to prevent players from falling

Using a solid node as mapgen_singlenode is a tried-and-tested technique employed by CTF to prevent players from falling if they somehow end up outside the map. I'm sure it will be just as useful for ITB, if not more.

Needs to be tested to ensure that this does indeed prevent players from falling, and more importantly, whether players can walk around in the lobby and that the air within the boxes has been left intact.