mod_http_oauth2: Reject invalid attempt to register client without credentials The implicit flow works without a client_secret since the token is delivered directly, but all other currently supported grant types require client to authenticate using credentials, so it makes no sense to not issue credentials then.

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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.