Fix placing a tent in a car

Fixes #9318

Adds checks for creatures (npc/monster) and for vehicles. It also adds checks for items (`game::is_empty`) because otherwise the walls of the tent could be placed on items and those items can not be accessed via the adv. inventory.

The function `game::creature_at` is not implemented (I tried that first, linked complained), but the function `game::critter_at` is.
I'm too indecisive: I had it in, than remove it as redundant, than I added it back for the item check. It's good good that you check this stuff. It's fixed or you can fix it with the merge.