Overlay fixing and atmos magboots.

Created by: ComicIronic

Special magboot reskins for atmos techies. Feel special today! Magboot toggle code made more sensible. Humans now properly update to hide and show icons that are hidden by other equipped items. I would like to unite hidden flags at some point, but the current system works fine enough. Fixes #2406 (closed). (NB: atmos magboot sprites shamelessly stolen from custom_items.dmi)