mod_muc_bot: Save occupant to room This has some side-effects: Firstly, the bot shows up in occupant list, which is nice. Secondly, the bot starts receiving messages from the room which might be wanted, but it would be better to join the room for real in this case.

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.