be-mbox-to-xml is now better at message-id, in-reply-to, and references.

be-mbox-to-xml is now better at message-id, in-reply-to, and references.

A previous "len(ret) >= 0" had been stripping the alt-id and
in-reply-to from _all_ parts of multipart comments.  Now it only
strips them from parts after the first.  The following parts do not
specify and alt-id, and they all are in-reply-to the first part.

I also added the KNOWN_IDS list for selecting amongst an array of
possible in-reply-to or references ids.  This works well enough for
now, but would be more robust if we could import a list of previously
known ids from BE...