convert the assistant to use a bound thread for XMPP

This *may* solve the segfault I was seeing when the XMPP library called
startTLS. My hypothesis is as follows:

* TLS is documented
  (http://www.gnu.org/software/gnutls/manual/gnutls.html#Thread-safety)
  thread safe, but only when a single thread accesses it.
* forkIO threads are not bound to an OS thread, so it was possible for
  the threaded runtime to run part of the XMPP code on one thread, and
  then switch to another thread later.

So, forkOS, with its bound threads, should be used for the XMPP thread.
Since the crash doesn't happen reliably, I am not yet sure about this fix.

Note that I kept all the other threads in the assistant unbound, because
bound threads have significantly higher overhead.

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