allow a parent function to restrict use of placebc

allow a parent function to restrict use of placebc

placebc was triggering an impossible sometimes on the plane of
water

It turned out to be because movebubbles issued an
unplacebc(), but a downstream function called
placebc(), so when movebubbles() issued its own
placebc() there was a problem.

The downstream function that beat movebubbles to the placebc()
turned out to be unstuck(). There could be others.