networkd: add support for resolved hook for DHCP server

Let's synthesize DNS RRs for leases handed out by our DHCP server. This
way local VMs can have resolvable hostnames locally.

This does not implement reverse look ups for now. We can add this
later in similar fashion.

src/network/networkd-network.c

@@ -761,6 +761,7 @@ static Network *network_free(Network *network) {
                 free(network->dhcp_server_emit[t].addresses);
         ordered_hashmap_free(network->dhcp_server_send_options);
         ordered_hashmap_free(network->dhcp_server_send_vendor_options);
+        free(network->dhcp_server_local_lease_domain);
 
         /* DHCP client */
         free(network->dhcp_vendor_class_identifier);