Additional section processing improvements.

Now adding additional A/AAAA records for NS and MX records in
  answer section.
Rewritten ns_put_additional() - now puts only the desired records.
Zone node API for saving and getting node references cleaned up:
 - zn_add_ref_*() and zn_get_ref_*() replaced by generic functions
   zn_add_ref() and zn_get_refs().
 - redundant union members ns and mx replaced by generic member
   additional.