Support PRIVATEOID/PRIVATEDNS in zone.c

Support PRIVATEOID/PRIVATEDNS in zone.c

- dns_zone_cdscheck() has been extended to extract the key algorithms
  from DNSKEY data when the CDS algorithm is PRIVATEOID or PRIVATEDNS.

- dns_zone_signwithkey() has been extended to support signing with
  PRIVATEDNS and PRIVATEOID algorithms.  The signing record (type 65534)
  added at the zone apex to indicate the current state of automatic zone
  signing can now contain an additional two-byte field for the DST
  algorithm value, when the DNS secalg value isn't enough information.