Fix discovery in the absence of listen addresses (fixes #4418)

This makes it OK to not have any listeners working. Specifically,

- We don't complain about an empty listener address
- We don't complain about not having anything to announce to global
  discovery servers
- We don't send local discovery packets when there is nothing to
  announce.

The last point also fixes a thing where the list of addresses for local
discovery was set at startup time and never refreshed.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4517

lib/discover/global.go

@@ -208,8 +208,10 @@ func (c *globalClient) sendAnnouncement(timer *time.Timer) {
 	}
 
 	if len(ann.Addresses) == 0 {
-		c.setError(errors.New("nothing to announce"))
-		l.Debugln("Nothing to announce")
+		// There are legitimate cases for not having anything to announce,
+		// yet still using global discovery for lookups. Do not error out
+		// here.
+		c.setError(nil)
 		timer.Reset(announceErrorRetryInterval)
 		return
 	}