move DNS64 implementation into an external module

move DNS64 implementation into an external module

- dns64 implementation has been moved from query.c to the dns64 module.
- the code in lib/dns/dns64.c has not yet been moved.
- the module does not yet parse dns64 options in named.conf; that's
  still done by named.
- the module does not have persistent storage; we still use the client
  object.
- made more functions globally accessible so they can be called from
  modules: ns_query_lookup(), ns_query_addsoa(), ns_query_nodata(),
  ns_query_ncache(), ns_query_setorder().