OPT RR, real size of AXFR, stopping AXFR on error

- OPT RR adding to normal responses.
- OPT RR adding to AXFR. TODO: must somehow enforce larger packets
  than allowed by the OPT RR.
- Stopping AXFR when any error occurs (do not try to send NSEC3s).
- Real size of AXFR packets should not rewrite the buffer size in
  ns_xfr_t.
- Added a lot of debug output.