prismatic double-ball joint

This joint works like a slider with two ball-joints on each end, like one might find in a suspension damper of a car or motorcycle. It is in essence a single limit motor which is specified not in terms of its axis but in terms of two user-settable anchor points and therefore includes both linear and angular components. Patch is made against r1891 and includes an (admittedly weird) demo.

(submitted by Dimitris Papavasiliou)