Vehicle trunks are not stasis chambers

Fixes #4194 and a bunch of other issues that aren't even reported yet:

* Active items in vehicle CARGO parts will now be updated.
* game::find_item looks in vehicles for items now
* Refactored active item processing to avoid copypasta (there's ANOTHER copy of active item processing logic in player.cpp but that's another issue entirely)

This fixes explosive timers stopping in cars, food in cars staying hot forever, corpses in cars never reviving, and so on and so forth.