Adds check for threshold status and calls off the turret

for clearly non-human characters.

Incentive to not take Alpha, too, since those still pass for human.  BMG turrets are designed for vehicles so can target HUGE folks.  Tank turret is handled in the multi_robot scripting.
I'm a little concerned that people will get the impression that they are not and will never be a target based on the message, but I don't have a good alternative at the moment.