mod_sasl2_fast: Restore non-FAST SASL handler upon reset This fixes an issue where, after failed FAST authentication, the FAST handler was still used (even though the client now wants to try with a password or other non-FAST credentials). A little hacky perhaps, but probably the best we can do without baking FAST support directly into mod_sasl2.

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.