acid vs iron bars (trunk only)

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.

doc/fixes35.0

@@ -278,6 +278,7 @@ javelins and spears now share the same weapon skill
 all stackable weapons are capable of being thrown/shot for multi-shot volleys
 worm teeth and crysknives have become stackable
 improved container interface
+acid can destroy iron bars
 
 
 Platform- and/or Interface-Specific New Features