mod_pubsub_subscription: support subscribing from a bare JID Allow subscribing from a bare JID on the component instead of only the component host, useful for subscribing to whitelist access model nodes that want to see a particular JID in the from.

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.