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/dg_scripts.h

@@ -71,6 +71,7 @@
 #define MTRIG_DOOR             (1 << 17)     /* door manipulated in room   */
 
 #define MTRIG_TIME             (1 << 19)     /* trigger based on game hour */
+#define MTRIG_DAMAGE           (1 << 20)     /* trigger whenever mob is damaged */
 
 /* obj trigger types */
 #define OTRIG_GLOBAL           (1 << 0)	     /* unused                     */
@@ -264,6 +265,7 @@ void time_wtrigger(room_data *room);
 
 int login_wtrigger(struct room_data *room, char_data *actor);
 
+int damage_mtrigger(char_data *ch, char_data *victim, int dam, int attacktype);
 /* function prototypes from dg_scripts.c */
 ACMD(do_attach) ;
 ACMD(do_detach);