Re-enables infections, implements a wound infection system

This PR re-enables incrementing germ_level for carbon mobs, which was previously commented out. This meant that mobs never had germ_levels above 0, so it seems you could actually do surgery without washing your hands, as you would never have any germs to transfer.

This also builds on the partially-implemented wound infection system. Untreated wounds can very slowly become infected. Moving around makes it happen a bit faster. The rates might need more tweaking. I was kind of afraid to make infection happen too fast - so in it's present state the system might happen too slowly for anyone to notice.