When giving items between avatars in different simulators, only add the item to the receiving avatar's inventory once.

When a user gives an item, the user's client sends an InventoryOffered IM message to its simulator. This adds the item to the receiver's inventory. If the receiver isn't in the same simulator then XMLRPC is used to forward the IM to the correct simulator. The bug was that the receiving simulator handled the message by calling OnInstantMessage() again, which added a second copy of the item to the inventory. Instead, the receiving simulator should only notify the avatar that the item was offered.
minor: Recomment out log message uncommented in previous cbdfe969