[buddy] avoid cache errors in bdd_satcount() and friends
* src/bddop.c (bdd_satcount, bdd_satcountln): If the number of declared variables changed since we last used it, reset misccache. Otherwise, bdd_satcount() and friends might return incorrect results after the number of variable is changed. These is needed for the next patch in Spot to pass all tests. (misccache_varnum): New global variable to track that. (bdd_satcountset): Make sure that bdd_satcountset(bddtrue, bddtrue) return 1.0 and not 0.0.