Don't allow users to attempt to sit on objects in a child region without going to that region first.

If this is attempted, they get a "Try moving closer.  Can't sit on object because it is not in the same region as you." message instead, which is the same as current ll grid.
Sitting on ground is okay, since viewer navigates avatar to required region first before sitting.
Implement ability for hg logins to try fallback regions just like local logins.

These would be specified in the [GridService] section of Robust.HG.ini, which already lists these in the example text.
Untested patch so that Neb can easily pull in for testing, though shouldn't disrupt existing hg logins since fallback processing is a bit of code stuck on the end of the login sequence.