1.0.19.3: more careful PROGV and SET

1.0.19.3: more careful PROGV and SET

 * Don't bind constants in PROGV.

 * Check variable types before binding / assignment.

 * When un-binding, PROGV doesn't temporarily bind a variable to NIL
   anymore, but directly to the unbound marker, so that an interrupt
   handler cannot see a bogus value.

 * Based on patch by Richard Kreuter.