simplification: search on rpz_st.p_name whenever RPZ is in use

simplification: search on rpz_st.p_name whenever RPZ is in use

- this is part of an effort to reduce interdependency between dns64 and
  other features, so that dns64 can more easily be compartmentalized
  into a module.
- this change keeps us from having to know whether dns64 is in use
  when updating the name to be searched for in query_lookup() because
  of RPZ processing. it seems to have no ill effects.
- also incorporated the test case from the CVE-2017-3135, which related
  to simultaneous use of dns64 and rpz.