fix #H4057 - rusting amulets

There have been two or three reports on getting feedback about
amulets rusting.  Object formatting doesn't display erosion for
them, so being told about damage then not seeing that damage
feels like a bug.  Even if damage was displayed, it has no effect
on them so would still feel somewhat strange.  It does display
erosion for wands and rings, which is strange too.

This limits erosion damage--and its feedback--to items which are
actually impacted by erosion:  armor, weapons and weapon-tools;
also heavy iron balls and iron chains since they've traditionally
shown rust even though it has little effect.

A side-effect of this change is that flammable items (other than
armor and weapons) which don't burn up immediately will no longer
become burnt, then very burnt, thorougly burnt, and finally be
destroyed.  Since the player couldn't see or possibly repair the
erosion state, it seemed incomplete.  It could be reinstated by
making other flammable items be subject to erosion and displayed
as such by xname() & co.

Wishing now avoids applying erosion and erosion-proofing to items
that aren't affected by it, regardless of material.  It also now
allows wishing for "rusty rustproof <iron-object>" which used to
suppress "rusty" in that combination and triggered a couple of
old bug reports.

Heavy iron balls and iron chains can have rust repaired and can
be made rustproof by wielding, then reading enchant weapon while
confused, as if they were weapons.

doc/fixes36.1

@@ -182,6 +182,8 @@ when looting multiple containers, add 'n' for "loot next container", change
 change looting to choose ":iobrs nq" action from a menu for menustyle:Partial
 wand/spell/breath zaps that reached the edge of the level on the Plane of Air
 	and "vanished into the aether" left temporary display effects on screen
+stop amulets and other items which aren't affected by erosion damage from
+	being subjected to erosion damage
 
 
 Platform- and/or Interface-Specific Fixes