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_test.go

@@ -214,7 +214,7 @@ func TestGlobalAnnounce(t *testing.T) {
 	}
 
 	if !strings.Contains(string(s.announce), "tcp://0.0.0.0:22000") {
-		t.Errorf("announce missing addresses address: %s", s.announce)
+		t.Errorf("announce missing address: %q", s.announce)
 	}
 }