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.

include/decl.h

@@ -431,6 +431,7 @@ struct plinemsg_type {
 
 E struct plinemsg_type *plinemsg_types;
 
+enum bcargs {override_restriction = -1};
 struct breadcrumbs {
     const char *funcnm;
     int linenum;