Added a new trigger type for mobs, called "Damage", that triggers every (#151)

time the mob is harmed, through any means.

Valid return values: 
-1: prevents damage from occurring. Will also prevent a fight from
starting.
0: forces a miss.
>0 : the damage the mob will endure.

Available variables:
%actor%: the one doing the damage
%victim%: typically the same as %self% - the one being attacked
%damage%: the damage inflicted. Always a non-negative number.
%attacktype%: The attack type. Will be UNDEFINED when hitting with a
weapon.

src/fight.c

@@ -620,6 +620,11 @@ int damage(struct char_data *ch, struct char_data *victim, int dam, int attackty
   if (!IS_NPC(victim) && ((GET_LEVEL(victim) >= LVL_IMMORT) && PRF_FLAGGED(victim, PRF_NOHASSLE)))
     dam = 0;
 
+  dam = damage_mtrigger(ch, victim, dam, attacktype);
+  if (dam == -1) {
+  	return (0);
+  }
+
   if (victim != ch) {
     /* Start the attacker fighting the victim */
     if (GET_POS(ch) > POS_STUNNED && (FIGHTING(ch) == NULL))