Nameserver answering + changes to zn and zdb.

Added functions:
  - ns_get_zone_for_qname() which returns proper zone for searching
    (special handling of DS queries),
  - ns_find_node_in_zone() which repeatedly strips leftmost label
    from the QNAME and searches in the zone,
  - ns_follow_cname() which resolves the CNAME chain and puts all
    CNAME records to the response packet,
  - ns_copy_answers() which puts the appropriate RRSet from a node
    to the response packet.

Improved the ns_answer() function. Some cases remain TODO:
  - referral
  - adding SOA when name not found in zone
  - DNAME
  - wildcard

zdb_find_zone_for_name() made public and zdb_find_name() changed
  to zdb_find_name_in_zone().
Sorting of zones in zone database in reverse canonical order
  (added function zdb_insert_zone() taking care of this).

Added members to zn_node:
  - flags
  - CNAME pointer
Added functions to zone node to get/set delegation point flag and
  to check if the node has CNAME.