resolved: rework mDNS cache-flush bit handling

resolved: rework mDNS cache-flush bit handling

This adds a new DnsAnswer item flag "DNS_ANSWER_SHARED_OWNER" which is
set for mDNS RRs that lack the cache-flush bit. The cache-flush bit is
removed from the DnsResourceRecord object in favour of this.

This also splits out the code that removes previous entries when adding
new positive ones into a new separate call dns_cache_remove_previous().