mod_sasl2_fast: Fixes to make channel binding work again tls-endpoint isn't a thing that exists. Also, we needed to copy more channel binding state from the primary sasl_handler. Ideally we'd have a cleaner way to do this, but I think that's part of more substantial changes that the SASL API deserves.

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.