- multiple DLZ's can be specified, including multiple DLZ's using
   the same driver; e.g., two different back-ends both loaded by the
   dlopen driver
 - new "search" option can be specified in a DLZ indicating whether
   this DLZ database should be searched for unknown zones.  The
   default is "yes".  If "no", then the zone can only be found by
   named if it's registered in the zone table, which happens if the
   zone is configured for dynamic updates, or if "dlz <dlzname>" is
   specified in the zone statement. (The latter functionality is
   incomplete in this commit).