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