Fix issue where only one of multiple attachments on the same attachpoint for NPCs would be seen by other viewers.

It appears that at least Singularity 1.8.5 (but probably others) rely on attachment FromItemIDs being different to display more than one.
This commit resolves this by generating random IDs instead of always using UUID.Zero for NPCs.
Resolves http://opensimulator.org/mantis/view.php?id=7110