Inventory script fixes

This was tested with the bk_treasuryreport item.
Adds support for PcSkipEquip special variable.
There are some oddities in MW's script engine needed to make this work:
 - OnPcEquip needs to be set on *using* any item, not just equipping
 - Immediately after setting OnPcEquip, the item's script needs to be executed once.
 - Do not set OnPcEquip when an item that has skipped equipping sets pcskipequip back to 0 and gets equipped