mod_bookmarks2: Add new module. This is the result of hacking during the Stockholm XMPP Sprint, for compatibility with older clients only doing Private XML XEP-0048. This module shouldn’t be loaded at the same time as mod_bookmarks, as both implement Private XML to achieve their conversion.

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.