Michał Kępień authored Oct 09, 2018 and Ondřej Surý committed Oct 24, 2018

Rather than overloading dns_zone_slave and discerning between a slave
zone and a mirror zone using a zone option, define a separate enum
value, dns_zone_mirror, to be used exclusively by mirror zones.  Update
code handling slave zones to ensure it also handles mirror zones where
applicable.