fix #K3455 - rocks vs xorns

Implement the suggestion that falling rock traps and rolling boulder
traps be harmless to xorns.  I've extended that to all missiles made
of stone (rocks, gems, boulders, a handful of other things that will
only matter if poly'd hero throws in '<' direction or is hit by stuff
scattered by an explosion).

I excluded ghosts because they would become even harder to kill and
the missile handling would need extra checks to test for blessed objs.

include/obj.h

@@ -226,6 +226,12 @@ struct obj {
 #define uslinging() (uwep && objects[uwep->otyp].oc_skill == P_SLING)
 /* 'is_quest_artifact()' only applies to the current role's artifact */
 #define any_quest_artifact(o) ((o)->oartifact >= ART_ORB_OF_DETECTION)
+/* 'missile' aspect is up to the caller and does not imply is_missile();
+   rings might be launched as missiles when being scattered by an explosion */
+#define stone_missile(o) \
+    ((o) && (objects[(o)->otyp].oc_material == GEMSTONE             \
+             || (objects[(o)->otyp].oc_material == MINERAL))        \
+         && (o)->oclass != RING_CLASS)
 
 /* Armor */
 #define is_shield(otmp)          \