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.

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.