Replace the "mirror" zone option with "type mirror;"

Replace the "mirror" zone option with "type mirror;"

Use a zone's 'type' field instead of the value of its DNS_ZONEOPT_MIRROR
option for checking whether it is a mirror zone.  This makes said zone
option and its associated helper function, dns_zone_mirror(), redundant,
so remove them.  Remove a check specific to mirror zones from
named_zone_reusable() since another check in that function ensures that
changing a zone's type prevents it from being reused during
reconfiguration.