Cleans up organ processing

(cherry picked from commit 1b86e3548fbe6c3ad8cd7fa6d124609ee3654ae6)
Infections now spread between organs

Infections can now spread between internal and external organs.
Also changes the organ_failure random event to cause an infection
instead.

(cherry picked from commit 244b43c657e6e9e53c7c35a965d6252a37c1b01b)

Conflicts:
	code/setup.dm