Make yellow dragon breath destroy iron bars (unless their wall_info
is marked as non-diggable, which I think is currently impossible for bars).
The shop handling is untested; no shop has iron bars for its walls or
includes them as interior obstacles. (The latter would require changes
to shop repair because lost bars will become solid wall if/when rebuilt.)
Thrown potions of acid and spat or thrown splashes of acid venom
don't affect bars--at least so far--because those always pass through
without hitting. Monsters who have a non-ranged attack which does acidic
damage can't use such attacks against iron bars.
e7cb81fe7b