mod_http_oauth2: Don't issue client_secret when not using authentication This is pretty much only for implicit flow, which is considered insecure anyway, so this is of limited value. If we delete all the implicit flow code, this could be reverted.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.