Fix recent regression from 473c5594 where camera started to judder on moving vehicles.

Other parts of OpenSimulator are relying on SP.Velocity == 0 for vehicles.
So add and use SP.GetWorldVelocity() instead when we need vehicle velocity, along the same lines as existing SP.GetWorldRotation()
Add [EntityTransfer] AllowAvatarCrossing setting to determine whether avatars are allowed to cross regions at all.

Defaults to true.  For test purposes.