Free enemy knowledge issues / Ambush changes

[TotallyMehis]

The knowledge is based on hearing, but there are some problems.

• Immolator will ignite even when it can't see the enemy.
• Zombies will attempt to walk through doors even when it can't see the survivor.

I think the latter is fine, but it can be annoying. I'd rather the zombies have some awareness of their surroundings. For this reason, I think changing the ambush to be vision based would circumvent this problem.

Proposed: Fix immolator igniting. Make ambush vision based. ie. All zombies in ambush "share" vision. If one of them sees a survivor, the ambush is activated.

Related to #221