resolve/mdns: do not append goodby packet entries to known answers section

When we receive a goodby packet about a host, and we have a cache entry about
the host, we do not immediately remove the cache entry, but update it with TTL 1.
See RFC 6762 section 10.1 and 3755027c2c.

If we receive a request soon after the goodby packet, previously the
entry was included in the known answers section of the reply. But such
information should not be appended.

Follow-up for 3755027c2c.

src/resolve/resolved-dns-cache.c

@@ -1303,6 +1303,10 @@ int dns_cache_export_shared_to_packet(DnsCache *cache, DnsPacket *p, usec_t ts,
                         if (!j->shared_owner)
                                 continue;
 
+                        /* Ignore cached goodby packet. See on_mdns_packet() and RFC 6762 section 10.1. */
+                        if (j->rr->ttl <= 1)
+                                continue;
+
                         /* RFC6762 7.1: Don't append records with less than half the TTL remaining
                          * as known answers. */
                         if (usec_sub_unsigned(j->until, ts) < j->rr->ttl * USEC_PER_SEC / 2)