fix #4206 - feedback for pet's weapon

fix #4206 - feedback for pet's weapon

The report that a tame Archon got two "<pet>'s long sword is not
affected" messages thought there was some duplication error when a
flaming sphere exploded, which was incorrect.  Since an Archon has
two weapon attacks, getting that message twice just meant that both
attacks hit.  However, the player has only 1/6 chance to suffering
passive fire damage to weapon, where monster-on-monster or monster-
on-polyd-hero was inflicting that for every successful hit, so
there was a bug here after all.  Give monsters the same 1/6 chance.
Also, add even more verbosity to that message--now that it won't be
delivered so often--to mention what didn't affect the item.

While investigating this, I noticed that hitting a steam vortex
with a flammable weapon was doing fire damage to that weapon.  Fire
damage in the steam vortex definition makes some sense in that it
allows fire resistance to give protection, but dishing out actual
fire damage makes no sense and is now prevented for passive counter-
effects.