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_olc.h

@@ -16,7 +16,7 @@
 
 #include "dg_scripts.h"
 
-#define NUM_TRIG_TYPE_FLAGS		20
+#define NUM_TRIG_TYPE_FLAGS		21
 
 /* Submodes of TRIGEDIT connectedness. */
 #define TRIGEDIT_MAIN_MENU              0