[InterNetNews/inn] 4fe713: Fix the use of Auto-Submitted header field

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: 4fe713d0e57c42b993f6c0915baad355ff43c269
      https://github.com/InterNetNews/inn/commit/4fe713d0e57c42b993f6c0915baad355ff43c269
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2023-01-31 (Tue, 31 Jan 2023)

  Changed paths:
    M backends/news2mail.in
    M control/controlchan.in
    M doc/pod/inn.conf.pod
    M doc/pod/newsfeeds.pod
    M nnrpd/post.c

  Log Message:
  -----------
  Fix the use of Auto-Submitted header field

Use auto-generated for controlchan reports, and do not use that header
field at all for gatewaying (news2mail and nnrpd).

Thanks, Russ Allbery, for your wise thoughts:

The notifications sent by controlchan are not an autoreply to the
sender of the control message.  They are a status report sent to the
news administrator about a change made (or proposed) on their news
server by a third party.  As such, they're equivalent to the daily
report or the notification messages from innwatch.  The fact that the
change to the server was initiated by a netnews message doesn't feel
horribly relevant.  A "reply" by definition goes back to the sender
of the message; notification messages that go to third parties feel
inherently different.

Gatewaying (of which moderation is a subset) is yet a different thing,
and the Auto-Submitted header isn't appropriate here with any value.
These messages are not Auto-Submitted; they're, well, gatewayed.
This is equivalent to email forwarding; mail servers don't add
Auto-Submitted headers when they forward mails.

*If* we were encapsulating messages when sending them to moderators,
"Auto-Submitted: auto-generated" may be in the outer envelope, but
since we're transforming and forwarding a message, this is just part
of the message propagation path and thus should not be labeled with
Auto-Submitted.  The message is not, in general, auto-submitted; it was
manually posted by the poster of the message, and we're just passing it
along to its ultimate recipient.

Also, on a more practical level, addition of this header is
potentially surprising to moderators.  Their mail client may rightfully
downgrade or refile messages with Auto-Submitted headers, particularly
Auto-Submitted: auto-replied, since the vast majority of such messages
are vacation auto-replies that are of low importance or that people may
even auto-delete.  There's some risk of confusing submitted posts for a
moderated group, which are relatively important mail, for relatively
unimportant mail like vacation autoreplies or automated status
messages.

see #255