mod_unsubscriber: Add MUC support This catches XEP-0045 Join stanzas and replies with an error telling the prospective participant that the supposed MUC is gone. The user or their client can then take action to remove any bookmark or at least disable autojoin.

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.