network: add NextServer= and Filename= setting to [DHCPServer] section

Closes #4403.

src/network/networkd-network.c

@@ -700,6 +700,7 @@ static Network *network_free(Network *network) {
 
         free(network->dhcp_server_relay_agent_circuit_id);
         free(network->dhcp_server_relay_agent_remote_id);
+        free(network->dhcp_server_filename);
 
         free(network->description);
         free(network->dhcp_vendor_class_identifier);