Fixes porta turrets from infinitely stunning you.

- Prone mobs are not valid targets for stun turrets.

Turret behaviour:
- Turrets close when they don't have valid targets, but stay open if they can line up another shot after their cooldown is over, like before.
- Handcuffed mobs are not valid targets.
- Mobs buckled to chairs are valid targets. Because they can not be prone, they are always valid stun targets, so you can bypass stun turrets with a buckled-but-not-handcuffed friend or two.

Fixes #5227.