Function ns_follow_cname() generalized to put RRs to given section.

Once again putting RRSets to packet separated into generic function
  ns_put_rrset(), now with wildcard processing.

ns_put_additional() now puts all additional RRSets into the packet,
  not only for given type (either way was iterating through the
  whole Answer section).
ns_put_additional() now processes CNAMEs.

zdb_adjust_additional() now saves also pointer to node with CNAME
  record, to be used in answering.

Structure zn_ar_rrsets modified to be able to hold pointer to CNAME
  node. TODO: consider using union to save space.

Added zn_add_ref_cname() to add CNAME additional reference.
  TODO: is copy-pasted from zn_add_ref(), consider refactoring.