fix #K2617 - metallivores eating iron bars

Monsters with rust attacks (rust monster) and corrosion attacks
(black pudding, gray ooze) can eat or otherwise destroy iron bars
but xorns could only move through the iron bars spot without being
able to eat the metal there.  Change xorn to eat bars instead of
phazing through them.  Lets rock moles eat bars too.

Hero polymorphed into a rust monster would eat bars if trying to
move to their location but couldn't do so if already there (maybe
was in xorn form and now in rust monster form).  Xorns could pass
through them but not eat them.  Allow hero metallivores to eat
bars at the current location via 'e', similar to eating food off
the floor.  Hero as rock mole behaves like rust monster.

doc/fixes37.0

@@ -1,4 +1,4 @@
-NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.332 $ $NHDT-Date: 1602958104 2020/10/17 18:08:24 $
+NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.337 $ $NHDT-Date: 1603507384 2020/10/24 02:43:04 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -280,6 +280,12 @@ when reporting that hero can't repair a chest's broken lock with key/pick/card
 pre-populate teleport destination prompt with travel destination
 ghosts cannot be renamed
 tossed upwards objects got two times half physical damage reduction
+monster xorns could pass through iron bars but not eat them; monster rock
+	moles could no neither; now they can eat bars when adjacent and will
+	do so if the bars are blocking their path
+hero poly'd into rust monster could implicitly eat bars when adjacent by
+	trying to move there, now when in rock mole form too; in xorn form
+	can explicitly eat them via 'e' after moving onto their spot
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository