https://datatracker.ietf.org/doc/html/rfc6762#section-5.4

When receiving a question with the unicast-response bit set, a
responder SHOULD usually respond with a unicast packet directed back
to the querier. However, if the responder has not multicast that
record recently (within one quarter of its TTL), then the responder
SHOULD instead multicast the response so as to keep all the peer
caches up to date, and to permit passive conflict detection. In the
case of answering a probe question (Section 8.1) with the unicast-
response bit set, the responder should always generate the requested
unicast response, but it may also send a multicast announcement if
the time since the last multicast announcement of that record is more
than a quarter of its TTL.

Unicast replies are subject to all the same packet generation rules
as multicast replies, including the cache-flush bit (Section 10.2)
and (except when defending a unique name against a probe from another
host) randomized delays to reduce network collisions (Section 6).

This means additionals are properties of the answers and we only multicast them if we are multicasting the answer as well.

Apple's mDNSResponder behaves this way in testing so lets do that
fixes #689