dlna: add SSDP AnnounceInterval flag option

The current default AnnounceInterval is too short, causing the
multicast domain to be flooded with NOTIFY announcements,
which may prevent other dlna devices from sleeping.

This change allows users to set the announcement interval,
and it's default value also increased to 12 minutes.

Even within the interval, rclone can still passively respond to
M-SEARCH requests from other devices.

cmd/serve/dlna/dlna.go

@@ -118,7 +118,7 @@ func newServer(f fs.Fs, opt *dlnaflags.Options) (*server, error) {
 	}
 
 	s := &server{
-		AnnounceInterval: 10 * time.Second,
+		AnnounceInterval: opt.AnnounceInterval,
 		FriendlyName:     friendlyName,
 		RootDeviceUUID:   makeDeviceUUID(friendlyName),
 		Interfaces:       interfaces,