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

Previous way of handling NOTIFY settings for mirror zones was a bit
tricky: any value of the "notify" option was accepted, but it was
subsequently overridden with dns_notifytype_explicit.  Given the way
zone configuration is performed, this resulted in the following
behavior:

  - if "notify yes;" was set explicitly at any configuration level or
    inherited from default configuration, it was silently changed and so
    only hosts specified in "also-notify", if any, were notified,

  - if "notify no;" was set at any configuration level, it was
    effectively honored since even though zone->notifytype was silently
    set to dns_notifytype_explicit, the "also-notify" option was never
    processed due to "notify no;" being set.

Effectively, this only allowed the hosts specified in "also-notify" to
be notified, when either "notify yes;" or "notify explicit;" was
explicitly set or inherited from default configuration.

Clean up handling of NOTIFY settings for mirror zones by:

  - reporting a configuration error when anything else than "notify no;"
    or "notify explicit;" is set for a mirror zone at the zone level,

  - overriding inherited "notify yes;" setting with "notify explicit;"
    for mirror zones,

  - informing the user when the "notify" setting is overridden, unless
    the setting in question was inherited from default configuration.