Fix llGetLinkKey() to return the last sat avatar as the last link number.

As per http://wiki.secondlife.com/wiki/LlGetLinkKey
This is done by keeping a scene-object wide list of sitters.
This also fixes bugs in this function where linknums 0 and 1 weren't treated properly if there were sitting avatars on a single prim.
This also fixes a minor race condition for multiple concurrent sitters on a prim with no current sitters by locking on the object-wide list rather than individual sop lists
Addresses http://opensimulator.org/mantis/view.php?id=6477