CS1411: Iron bars can be dissolved by potions of acid
CS1412: Give acid breath a chance to dissolve iron bars

1 in 5 chance, like thrown potions of acid.
CS1415: Metallivores eat through iron bars

Partly based on a patch by Malcolm Ryan.

This also allows monsters with disintegration attacks to dissolve iron
bars.

Known bug: The hero polymorphed into a disintegration dragon walks
freely through iron bars without affecting them, but enemy
disintegration dragons will dissolve them as they move through them.
CS1459: Destroying iron bars in Sokoban is cheating

... and monsters (mostly) won't do it for you.

zap_over_floor() does not discern if zapped acid is from the hero or
from enemies, so iron bars dissolved this way will increment the Sokoban
conduct count regardless of its origin.
Fix warning when trying to take off merged dragon skin

This bug also exists in upstream NitroHack.