To avoid viewers (particularly on the Hypergrid) from permanently caching a UUID -> "Unknown User" binding, drop the binding request rather than replying with "Unknown User"

By not binding UUID -> "Unknown User", we leave open the possibility that the binding may be correctly resolved at a later time, which can still happen in some Hypergrid situations.
Observed behaviour of LL viewer 3.3.4 is that a dropped bind request is not retried until the next session.
Make llGetLinkPrimitiveParams() abort and return existing list of params when it encounters an invalid link number, rather than throwing an exception

Addresses http://opensimulator.org/mantis/view.php?id=6768
Thanks to talun for the patch on that commit - in the end I took a different approach that also deals with invalid PRIM_LINK_TARGET
However, not yet generating the same warning on invalid PRIM_LINK_TARGET as seen on LL grid
This commit also adds regression tests for some cases of llGetLinkPrimitiveParams()
Fix bug where using PRIM_LINK_TARGET with only two remaining list items (e.g. link number then PRIM_ROTATION) would not return the parameter

Extended regression test for this case