serve dlna: fix SSDP discovery not working - fixes #10276

github.com/anacrolix/dms v1.8.0 added IPv6 SSDP support which requires
the multicast address to be set in ssdp.Server. rclone wasn't setting
it so creating the SSDP socket failed and the error was ignored,
meaning DLNA clients could no longer discover the server.

cmd/serve/dlna/dlna.go

@@ -482,7 +482,9 @@ func (s *server) ssdpInterface(intf net.Interface) {
 	// Note that the devices and services advertised here via SSDP should be
 	// in agreement with the rootDesc XML descriptor that is defined above.
 	ssdpServer := ssdp.Server{
-		Interface: intf,
+		Interface:  intf,
+		AddrString: ssdp.AddrString,
+		NetAddr:    ssdp.NetAddr,
 		Devices: []string{
 			"urn:schemas-upnp-org:device:MediaServer:1"},
 		Services: []string{