Updated SSU sprites and update_icon proc

Condensed the plethora of duplicate icons into just a few. Closed SSU sprites are now the same regardless of containing suit, helmet or neither but whatever, pretend the glass is tinted.

Made the SSU's update_icon proc into overlays.

Can't put items into a broken SSU anymore since they're not retrievable until it's fixed.

Tested quite a bit and I'm fairly confident all the states should be working fine, although that's not to preclude me forgetting something.