Bay dev updates + basic Infection mode WIP.

This will do for now. It's not finished but it won't break anything.

- [x] Implement basic zombie controller, biting, infection, zombification proc.
- [x] Implement basic game mode.
- [x] Add preferences option for BE_ZOMBIE.

Not done in this PR, still to come:
- [ ] Implement zombie castes.
- [ ] Balance zombies against aggressors.
- [ ] Shakedown/testing.