Fixes russian revolver being abusable. 

You can no longer use it on someone other than yourself. Fixes #4707 . Aiming somewhere other than head/eyes/mouth doesn't instakill but does deal damage (and the shooting message show that you weren't aiming at your head like you should). You need to spin the chamber after each shooting attempt. Only humans can use the gun.

Making a can_trigger_gun() proc that tests if the mob can trigger the gun.