resolved: add support for linked packets

For mDNS, we need to support the TC bit in case the list of known answers
exceed the maximum packet size.

For this, add a 'more' pointer to DnsPacket for an additional packet.

When a packet is unref'ed, the ->more packet is also unrefed, so it
sufficient to only keep track of the 1st packet in a chain.

src/resolve/resolved-dns-packet.c

@@ -170,6 +170,9 @@ DnsPacket *dns_packet_unref(DnsPacket *p) {
 
         assert(p->n_ref > 0);
 
+        if (p->more)
+                dns_packet_unref(p->more);
+
         if (p->n_ref == 1)
                 dns_packet_free(p);
         else

src/resolve/resolved-dns-packet.h

@@ -88,6 +88,9 @@ struct DnsPacket {
         uint16_t sender_port, destination_port;
         uint32_t ttl;
 
+        /* For support of truncated packets */
+        DnsPacket *more;
+
         bool on_stack:1;
         bool extracted:1;
         bool refuse_compression:1;