Hero polymorphed into a disintegration dragon can walk through iron bars without affecting them

[tung]

Exists as mentioned in bdfb9a5.

Enemy disintegration dragons will dissolve iron bars as they move through them, but the player polymorphed into one can walk through them without dissolving them.

It's not clear what the intended behavior for this should be:

1. The hero should dissolve the iron bars like enemy disintegration dragons can.
2. Only monsters with melee/non-breath disintegration attacks should be able to dissolve iron bars, and no disintegration dragons should be able to walk through them, only breathe on them to dissolve them.

This bug also exists in UnNetHack.