bug#28843: 26.0.90; gnus kills unsaved message buffer

[Nick Helm <nick@tenpoint.co.nz>]

When gnus is the mail-user-agent, quitting gnus causes unsaved messages
to be lost.

Emacs -Q
(setq mail-user-agent 'gnus-user-agent)
M-x gnus
C-x 5 m "xxx" ;make and modify a new message
C-x 5 b "*Group*" ;back to gnus
q, yes

Gnus exits, and the unsaved message buffer dies with it, without prompts
to save.


In GNU Emacs 26.0.90 (build 1, x86_64-apple-darwin16.7.0, NS appkit-1504.83 Version 10.12.6 (Build 16G29))
 of 2017-10-15 built on oberon
Windowing system distributor 'Apple', version 10.3.1504
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Emacs start-up time: 0.2 seconds

Configured features:
JPEG NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS LCMS2

Important settings:
  value of $LANG: en_NZ.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Text

Minor modes in effect:
  savehist-mode: t
  global-eldoc-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  visual-line-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr gnus-msg gnus-art mm-uu mml2015 mm-view mml-smime
smime dig mailcap gnus-sum gnus-group gnus-undo gnus-start gnus-cloud
nnimap nnmail mail-source tls gnutls utf7 netrc nnoo parse-time
gnus-spec gnus-int gnus-range gnus-win gnus nnheader wid-edit emacsbug
message rmc puny seq byte-opt bytecomp byte-compile cconv format-spec
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils time dired-x easymenu dired dired-loaddefs
pcase savehist nh-macdict easy-mmode iso-transl edmacro kmacro
cl-loaddefs cl-lib gv plain-theme time-date tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/ns-win ns-win
ucs-normalize mule-util term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote kqueue cocoa ns lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 262303 8886)
 (symbols 48 26274 4)
 (miscs 40 38 132)
 (strings 32 47796 1277)
 (string-bytes 1 1405572)
 (vectors 16 40422)
 (vector-slots 8 763529 24867)
 (floats 8 202 17)
 (intervals 56 206 0)
 (buffers 992 11))