Prevent quest items from being stashed, instead of items with no price

This lets us stash Scathelocke's Spellbook, which isn't a quest item and
has no price.
Allow multiple "req" lines per item
Change syntax for item requirements

Instead of "req=p,2", we now use "requires_stat=physical,2". Other stats
include: "mental", "offense", and "defense".
Merge pull request #1243 from dorkster/item_req

Item requirement improvements
Merge pull request #1237 from dorkster/fixes

Prevent quest items from being stashed, instead of items with no price