Dwarvish cloaks somewhat protect inventory from cold and fire

The change to Sokoban difficulty calculation introduced a balance
issue: previously the Sokoban prize was obtainable before item-
destruction attacks became common and would protect from them,
whereas now they commonly appear in Sokoban itself, before the
prize is accessible. This makes scrolls and potions much less
usable as escape items, and potions of healing less usable for
HP recovery, because they either get stashed or destroyed (and
in either case aren't usable by the character in time-critical
situations).

This commit attempts to alleviate the problem by adding a way to
protect items from cold and fire in the early game. It's a
little unreliable, and requires equipping an item that has bad
stats and generally isn't otherwise used, but which is generally
easy to find early on.

Not included in this commit, but probably necessary for a
"finished" version of the feature: some way of letting players
know the new mechanic. It shows up in enlightenment, but there
should probably be at least rumors (and maybe even a major Oracle
consultation) hinting at the mechanic. We might also want to
change the unIDed description of the dwarvish cloak as a hint,
although that would require, e.g., new database entries.

doc/fixes3-7-0.txt

@@ -616,6 +616,7 @@ yet another fix for display problems during restore:  if game is saved while
 	being obfuscated by hallucination, but when displaying the hero there
 	instead it would access steed pointer before that has been set up
 resistances gained from worn or wielded items also protect hero's inventory
+dwarvish cloaks somewhat protect hero's inventory from cold and fire
 non-metallic gloves protect worn rings from shock
 message "Oops!  food rations out of your grasp!" occurred due to perm_invent
 	in mid-operation overwriting all of xname's/doname's obufs; fixed by