Implements osForceSit(string avatar) & overload osForceSit(string avatar, string target)

Allows a script IN the target prim to force an avatar to sit on it using normal methods as if called by the client.

Overload method of osForceSit() to allow a script NOT in the target prim to force an avatar to sit on the target prim using normal methods as if called by the client.

This patch is based on previous work from
http://opensimulator.org/mantis/view.php?id=4492
and also includes the suggestions from justincc including change of threat level
Thank you Christos Lightling.
minor: spacing cleanup from previous commit 79a4d1ea