mod_pubsub_post: Restructure authentication and authorization (BC) This deprecates the default "superuser" actor model and makes the default equivalent to the previous "request.id". A single actor and secret per node is supported because HTTP and WebHooks don't normally include any authorization identity. Allowing authentication bypass when no secret is given should be relatively safe when the actor is unprivileged, as will be unless explicitly configured otherwise.

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.