Let bite attack be used against non-players

A step in the direction of monster (and NPC) infighting.

Uses zombie's current plans to determine target. If said target is in melee range and not friendly, the zombie will attempt to bite it like it would bite the player.

Can be seen by pitting an NPC against zeds or using pheromones.

Zombies with strictly anti-player attacks (like shockers) didn't get FRIENDLY_SPECIAL tag, so they can only bite when hostile, but they can bite NPCs.
You want to use `game::critter_at` here.