Name: check_killable - check if an enemy is killable by a living
Syntax:
int check_killable(object target)
Description:
The check_killable function checks if another living is killable by
this living. Players that don't have the killable flag set (see
make_killable) can't be killed by other players.
Return value:
1, if the target is killable by the living, 0 if not.
