silver vs poly'd hero (trunk only)

<Someone> reported that thitu() was adding d20 damage for silver object
hitting silver-hating hero even though all the callers were using dmgval()
which also does that, resulting in doubled silver bonus/penalty.  This
fixes that (including for boomerangs thrown by player, which weren't using
dmgval(), to handle a hyptothetical silver boomerang).  While testing it,
I noticed that there was no "the silver sears your flesh" message when a
monster hit you with a wielded silver weapon, so this fixes that too.
(How did we miss that?  And how did <Someone>? :-)

src/mthrowu.c

@@ -65,7 +65,7 @@ const char *name;	/* if null, then format `obj' */
 
 		if (obj && objects[obj->otyp].oc_material == SILVER
 				&& hates_silver(youmonst.data)) {
-			dam += rnd(20);
+			/* extra damage already applied by dmgval() */
 			pline_The("silver sears your flesh!");
 			exercise(A_CON, FALSE);
 		}