[DO NOT MERGE] Infection mode WIP.

Putting this up to keep track of the diff. Do NOT merge this, it will break like a crazy breaking thing and it's not even slightly finished.

- [ ] Implement basic zombie controller, biting, infection, zombification proc.
- [ ] Implement basic game mode.
- [ ] Implement zombie castes.
- [ ] Add preferences option for BE_ZOMBIE.
- [ ] Balance zombies against aggressors.
- [ ] Shakedown/testing.