mod_http_oauth2: Add a special "xmpp" scope that grants the users' default role This will be the first step towards defining a standard set of XMPP scopes. "xmpp" behaves as an alias for the user's default role, so that the client does not need to know about the various prosody:* roles.

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.