mod_sasl2_bind2, mod_sasl2_sm: Remove bind2 <features/> wrapper element
This was dropped from the spec. It's more consistent with 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.