mod_http_xep227: Initialize XEP-0227 XML mod_storage_xep0227 only writes if there is XML already for a user (it uses the presence of an existing <user> element as an indicator that an account exists, although technically this is not something Prosody itself does, so it's a little weird).

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.