Adjust avatar sit positioning on a target to pretty much exactly match that of the LL grid.

This uses an offset of 0.05 on the up vector of the sit orientation, after extensive analysis on
http://opensimulator.org/mantis/view.php?id=7096 and https://wiki.secondlife.com/wiki/Talk:LlSitTarget
This supersedes the previous adjustment in 5b73b9c4 which had been wrongly applied.
The maximum change is about 0.1 with a maximum height avatar.
This patch is all Aleric's work - I am applying manually in order to add these additional notes