- handle malformed answers from DLZ better:
 - handle dlz_lookup errors better:
   when the first lookup of a name returns an unexpected failure code,
   we return it to the caller rather than continuing on to look up
   the wildcard. we now only continue processing if the return from
   the first lookup was either ISC_R_SUCCESS or ISC_R_NOTFOUND.

 - improved backward-compatibility for dlz_version:
   added a DLZ_DLOPEN_AGE value indicating how many versions
   back from the current DLZ_DLOPEN_VERSION named will support