bug#17175: 24.3.50; continuing mail doesn't select "unsent mail" buffers

[Ken Olum]

If message-generate-new-buffers is 'unsent (the default in 24.3), then
messages being composed go in buffers beginning "*unsent mail".  But if
you try to continue sending mail, you don't get such buffers selected
but rather a new buffer created called "*mail*".

To reproduce:

emacs -Q
C-x m
C-u C-x m

                                        Ken

In GNU Emacs 24.3.50.4 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2014-01-06 on cosmos
Bzr revision: 115759 kdo@cosmos.phy.tufts.edu-20131227194350-g3o5h6bnyg8yntf2
Windowing system distributor `The Cygwin/X Project', version 11.0.10903000
System Description:     Ubuntu 12.04.4 LTS

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

Major mode: Lisp Interaction

Minor modes in effect:
  shell-dirtrack-mode: t
  diff-auto-refine-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent input:
M-x r e p o r t SPC e m a c s SPC b u g <return>

Recent messages:
Loading /home/kdo/emacs-init.el (source)...done
Loading /home/kdo/emacs-init-mail.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug sendmail rmailmm message
format-spec rfc822 mml mml-sec mm-decode mm-bodies mm-encode mailabbrev
gmm-utils mailheader mail-parse rfc2231 rmail rfc2047 rfc2045 ietf-drums
mm-util help-fns mail-prsvr mail-utils shell pcomplete diff-mode
easy-mmode thingatpt inf-lisp easymenu comint ansi-color ring warnings
cl-macs gv cl cl-loaddefs cl-lib time-date tooltip electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar dnd
fontset image regexp-opt fringe tabulated-list newcomment lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese case-table epa-hook jka-cmpr-hook help simple abbrev
minibuffer 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 make-network-process
inotify dynamic-setting font-render-setting x-toolkit x multi-tty emacs)