bug#37591: Ring big alarms when slipped a 'Mail-Copies-To: never' header

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

Big problem!

Let's say we get this mail

From: Robert Pluim <rpluim@gmail.com>
To: 積丹尼 Dan Jacobson <jidanni@jidanni.org>
Cc: Lars Ingebrigtsen <larsi@gnus.org>,  6615@debbugs.gnu.org,  34130@debbugs.gnu.org
Mail-Copies-To: never

Well, causally hitting S W to reply to all gives us a buffer with:

To: Lars Ingebrigtsen <larsi@gnus.org>
Cc: 6615@debbugs.gnu.org,  34130@debbugs.gnu.org
From: 積丹尼 Dan Jacobson <jidanni@jidanni.org>

Do you see the problem?

Gnus has blindly picked the **first** person from the CC list to become
the single To: person.

Yes, a proper mail needs a To: person. But it is a massive mistake to
pick one of the equal Cc: people to be the new To: person.

It was just like we were set up to redirect our anger at somebody else
via the silent booby trap Mail-Copies-To: never !

Solution: make all the people of the former Cc: and To: list now all equally
represented in a new single To:
To: Lars Ingebrigtsen <larsi@gnus.org>,  6615@debbugs.gnu.org,  34130@debbugs.gnu.org

Simple!

Furthermore, definitely give a warning:
'"Mail-Copies-To: never" header detected. Honor? [Y,n,?]'

And what if there were no Cc's or other To's ....

Anyway, the idea is to warn the user that he in now replying to someone else.
Also to eliminate making it look like we have chosen a special bystander
to now direct our wrath at instead of equally distributing it.

Gnus v5.13
GNU Emacs 26.3 (build 2, i686-pc-linux-gnu, GTK+ Version 3.24.11)
 of 2019-09-23, modified by Debian