trn's threading

Hi Team,

So while doing a little sleuthing last night, I discovered that trn
has a bunch of support for an antiquated threading data mechanism.

rt-mt.cpp: mt_init() has XTHREAD and raw thread file support
<https://github.com/LegalizeAdulthood/trn/blob/cmake/rt-mt.cpp#L117>

They both obtain the same (binary) data, one via NNTP and the other
via a local file.  XTHREAD is an old NNTP extension that delivers
threading information in a format specific to trn.  The OVER command
obsoletes the XTHREAD and XOVER commands in NNTP as described in RFC
3977 (Oct-2006).

I'm going to drop the code that supports these binary thread files,
whether obtained through NNTP or via local files.  My local news
server doesn't support XTHREAD, but I still have threading in trn, so
it must be getting thread information via XOVER in rt-ov.cpp.

I will switch the code to use the standard OVER command at some point.
First I need to add an "NNTP conversation" test harness mechanism to
be able to cover the existing NNTP implementation with tests before
making changes.

Cheers,

-- Richard

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