map.h: Fix small error in comment on map_space_t.
game: Add verious functions to check what can be constructed.

Add game_get_leveling_height().
Add game_can_build_military().
Add game_can_build_small(), game_can_build_mine(), game_can_build_large(), game_can_build_castle().
Add game_can_player_build().

interface: Implement determination of panel buttons and map cursor type in terms of these functions.
popup: Determine active buttons in popup construction boxes using these functions.