Add rule Aggro:NPCAggroMaxDistanceEnabled - enabled by default

If enabled, NPC's will drop aggro beyond 600 units or what is defined at the zone level

zone/mob.cpp

@@ -116,7 +116,7 @@ Mob::Mob(const char* in_name,
 		fix_z_timer_engaged(100),
 		attack_anim_timer(1000),
 		position_update_melee_push_timer(500),
-		mHateListCleanup(6000)
+		hate_list_cleanup_timer(6000)
 {
 	targeted = 0;
 	tar_ndx = 0;