fix #H7136 - iron bars vs non-diggable walls

Iron bars can be destroyed in some circumstances (hit by yellow
dragon breath or thrown potion of acid, being eaten by rust monser
or black pudding, or by poly'd hero in those forms) and should act
like walls for diggable/non-diggable purposes.  But they aren't
walls, so the non-diggable flag was not being set for them by the
special level loader.  Even once that was changed, they weren't
being handled consistently.  Some places checked for non-diggable
directly (zap_over_floor of acid breath, potion of acid hitting bars)
and started working as intended, others used may_dig() to check
non-diggable (poly'd hero attempting to eat iron bars) but it doesn't
handle iron bars, and still others didn't check at all (bars-eating
monster who moved onto bars location in expectation of eating those
next).

doc/fixes36.2

@@ -142,6 +142,8 @@ known bear trap was being forgotten about by a player polymorphed into a
 no leash-related message is given when a leashed pet yellow light explodes
 shop messages refer to shk by name even when shk is not visible but some
 	used pronoun "it" or "its" in same sentence; ditto for vault guards
+poly'd hero and monsters could eat through iron bars in areas where walls
+	were flagged as non-diggable
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository