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/mhitu.c

@@ -940,6 +940,9 @@ hitmu(mtmp, mattk)
 				artifact_hit(mtmp, &youmonst, otmp, &dmg,dieroll)))
 			     hitmsg(mtmp, mattk);
 			if (!dmg) break;
+			if (objects[otmp->otyp].oc_material == SILVER &&
+				hates_silver(youmonst.data))
+			    pline_The("silver sears your flesh!");
 			if (u.mh > 1 && u.mh > ((u.uac>0) ? dmg : dmg+u.uac) &&
 				   objects[otmp->otyp].oc_material == IRON &&
 					(u.umonnum==PM_BLACK_PUDDING