Improved zdb_insert_node_to_zone():
 - inserting into empty zone (must be SOA, checks if owner is
   the same as zone name)
 - checks if the node is not non-authoritative (subdomain of some
   delegation point), or delegation point (contains NS).
Some code from zdb_adjust_delegation_point() separated to
  zdb_extract_ns() and zdb_process_nonauth() (used elsewhere).

Added function zn_print_rrset().

WARNING: There's a BUG which somehow causes the data in zone nodes
         to be rewritten by data from the query!!