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

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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

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