mod_http_oauth2: Add support for the OpenID 'login_hint' parameter This allows the client to suggest to the authorization screen which user is trying to login, so they don't have to fill that in twice if they already did so at the client.

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.