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.

src/do_name.c

@@ -1728,6 +1728,9 @@ x_monnam(
             falseCap = (*pm_name != lowc(*pm_name));
     char *bp;
 
+    if (mtmp == &g.youmonst)
+        return strcpy(buf, "you"); /* ignore article, "invisible", &c */
+
     if (g.program_state.gameover)
         suppress |= SUPPRESS_HALLUCINATION;
     if (article == ARTICLE_YOUR && !mtmp->mtame)