Fix military bunker weirdness

Bunkers were odd because they shared the same omap ter, and the game checks the building above to see which one to spawn.  This caused errors in the z-level mod, as well as just being a terrible way of doing things.  A new omap ter was added for the basement level, and mapgen.cpp and overmap_special.json were modified to account for it.

Note: this breaks old saves.