<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>? :-)
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