mod_muc_moderation: Copy XEP-0421 occupant-id from retracted message Lets clients correlate the sender of whatever was retracted by moderators. Behavior limited to Prosody 0.12, otherwise there are no assurances of the origin of the occupant-id tag. Ref #1816

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.