another conflict abuse

Reported a really long time ago (June 2001) by <Someone>:
 - stand on the upstairs and engrave Elbereth with a /oFire
 - create Demogorgon and e.g. a tame dragon
 - cause conflict
 - Dragon will kill Demogorgon and Demogorgon will never ever attack Dragon.
All monsters could still attack Demogorgon without response via fightm().
Modified fightm() to include a bit of code in m_move and dog_move, allowing
response to an attack.  Testing this in action, Demogorgon still usually
did things detrimental to the player, mostly summoning nasty monsters.

src/mhitm.c

@@ -151,6 +151,16 @@ fightm(mtmp)		/* have monsters fight each other */
 		     */
 		    if (has_u_swallowed) return 0;
 
+		    /* Allow attacked monsters a chance to hit back. Primarily
+		     * to allow monsters that resist conflict to respond.
+		     */
+		    if ((result & MM_HIT) && !(result & MM_DEF_DIED) &&
+			rn2(4) && mon->movement >= NORMAL_SPEED) {
+			mon->movement -= NORMAL_SPEED;
+			notonhead = 0;
+			(void) mattackm(mon, mtmp);	/* return attack */
+		    }
+
 		    return ((result & MM_HIT) ? 1 : 0);
 		}
 	    }