Teach gnus/message about international Re: variants

Teach gnus/message about international Re: variants

* lisp/mail/mail-utils.el (mail-re-regexps): New defcustom,
contains the components used to construct 'rmail-re-abbrevs' and
'message-subject-re-regexp'.
* lisp/gnus/message.el (message-subject-re-regexp): Derive from
'mail-re-regexps'.
(message-strip-subject-re): Make the match case-insensitive.
* lisp/mail/rmail.el (rmail-re-abbrevs): Derive from
'mail-re-regexps'.  Update 'rmail-reply-regexp' when it changes.
(rmail-reply-regexp): Set to nil, 'rmail-re-abbrevs' will set
it.

* doc/emacs/rmail.texi (Rmail Reply): Describe
'mail-re-regexps'.
* doc/misc/message.texi (Message Headers): Describe
'mail-re-regexps'.

(Bug#72442)

(cherry picked from commit 315519fa7c8e5e142b6abca7b78a47ad3c0cbe37)