BUG: RBD: restraints: Corrected restraint force transformations

The restraints generate either joint-local (tau) or global (fx) forces.
At the moment they all generate the latter. This change corrects three
of the four restraints so that the forces are in the gobal coordinate
system and not the local coordinate system of the body.

The problem with this is that the forward dynamics code then transforms
most of the forces back to the body local coordinate system. A better
solution would be to associate restraints which are more sensibly
defined in a local frame with the joints instead of the bodies, and
return the forces as part of the tau variable.