U856 - cockatrice and nurses

One from the way-back machine.  A nurse would hit you-as-cockatrice repeatedly
and never turned to stone.  With this change, nurses will turn to stone (and
also don't heal cockatrices, which seems fair).  I considered giving them
gloves, but that seemed like too much effort.  There are other cases where a
monster "hits" but will not petrify.  However, it doesn't seem like passiveum
detects all the specific ways the monster "hit" you so I left them alone.

src/mhitu.c

@@ -1335,8 +1335,9 @@ dopois:
 		hurtarmor(AD_DCAY);
 		break;
 	    case AD_HEAL:
-		/* a cancelled nurse is just an ordinary monster */
-		if (mtmp->mcan) {
+		/* a cancelled nurse is just an ordinary monster,
+		 * nurses don't heal those that cause petrification */
+		if (mtmp->mcan || (Upolyd && touch_petrifies(youmonst.data))) {
 		    hitmsg(mtmp, mattk);
 		    break;
 		}