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.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.