PPPoE wasn't removed, which led to some refactoring to find
out why.  The outcome is the same, but works a little bit
different now.  While there, make it possible to omit the
config write in case we are going to call write_config()
shortly after anyway.

This will also be necessary for the legacy VPN plugins.

(cherry picked from commit ecdcc4b7)
(cherry picked from commit 3235ee26)