Make enhanced ammo last longer.

Since traps can no longer provide an infinite supply, and people were
complaining that they always ran out of ammo before, make it less likely
to break enchanted and blessed ammo, and more likely to break eroded
ones.

doc/fixes34.2

@@ -31,6 +31,7 @@ allow all tame monsters that eat to consider food thrown to them
 the screen display wasn't always up to date after map topology changes
 jumping over a sokobon pit would result in the player next to, not in, the pit
 don't let arrow, rock or dart traps provide an infinite number of objects
+make enhanced ammo harder to break to make lesser number last longer
 dropping from height or throwing a normal container may damage contents
 some Magicbane messages treated "erinys" as plural
 initialize artifacts before processing $WIZKIT

src/dothrow.c

@@ -1309,11 +1309,26 @@ register struct obj   *obj;
 		   sometimes disappear when thrown */
 		if (objects[otyp].oc_skill < P_NONE &&
 				objects[otyp].oc_skill > -P_BOOMERANG &&
-				!objects[otyp].oc_magic && rn2(3)) {
+				!objects[otyp].oc_magic) {
-		    if (*u.ushops)
+		    /* we were breaking 2/3 of everything unconditionally.
-			check_shop_obj(obj, bhitpos.x,bhitpos.y, TRUE);
+		     * we still don't want anything to survive unconditionally,
-		    obfree(obj, (struct obj *)0);
+		     * but we need ammo to stay around longer on average.
-		    return 1;
+		     */
+		    int broken, chance;
+		    chance = 3 + greatest_erosion(obj) - obj->spe;
+		    if (chance > 1)
+			broken = rn2(chance);
+		    else
+			broken = !rn2(4);
+		    if (obj->blessed && !rnl(4))
+			broken = 0;
+
+		    if (broken) {
+			if (*u.ushops)
+			    check_shop_obj(obj, bhitpos.x,bhitpos.y, TRUE);
+			obfree(obj, (struct obj *)0);
+			return 1;
+		    }
 		}
 		passive_obj(mon, obj, (struct attack *)0);
 	    } else {