store the zone configuration object in the zone

store the zone configuration object in the zone

when configuring a zone, we can now save the zone's configuration
object in the zone itself by calling dns_zone_setcfg().  this can
then be used by "rndc showzone" to print the zone's configuration,
which is simpler than searching for it using the new-zones
configuration, and allows it to work even if "allow-new-zones"
is disabled.