mod_bookmarks2: Fix handling of nick and password elements This form of child retrieval fails when the stanza elements internally don't have an 'xmlns' attribute, which can happen sometimes for some reason, including when they have been constructed via the stanza builder API. When that is the case then the explicit namespace arguemnt does not match the nil value of the internal attribute. Calling `:get_child()` without the namespace argument does the right thing here, with both nil and the parent namespace as valid values for the internal attribute.

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.