Add a major-mode for the *Messages* buffer

Add a major-mode for the *Messages* buffer

Ref: http://lists.gnu.org/archive/html/emacs-devel/2010-02/msg00135.html

* lisp/simple.el (messages-buffer-mode): New major mode.
(messages-buffer): New function.

* lisp/startup.el (normal-top-level): Switch mode of *Messages* buffer.

* src/xdisp.c (message_dolog): If we create *Messages*,
switch it to messages-buffer-mode.

* lisp/emacs-lisp/ert.el (ert--force-message-log-buffer-truncation)
(ert-run-test): Use `message-buffer' function.
(ert--force-message-log-buffer-truncation): Ignore read-only.

* lisp/help.el (view-echo-area-messages): Use `message-buffer' function.

* lisp/mail/emacsbug.el (report-emacs-bug): Use `message-buffer' function.

* lisp/gnus/gnus-util.el (gnus-message-with-timestamp-1):
Use `message-buffer' function if available.  Ignore read-only.

* etc/NEWS: Mention this.