bug#4455: 23.1.50; Can't turn off auto-fill-mode via mouse-minor-mode-menu

1. emacs -Q
2. Click mouse-3 over the major mode indicator in the mode line and
   select Auto fill from the pop-up menu.
3. Click mouse-1 over "Fill" in the mode line and select "Turn Off[sic]
   minor mode" from the pop-up menu.
=> Auto Fill mode remains enabled.

I traced this problem to auto-fill-function from simple.el, which is the
value of cmd in the last when-sexp of popup-menu from mouse.el: since
auto-fill-function is not interactive, (commandp cmd) is nil and the
body of the when-clause, containing a call to cmd, is not evaluated.
But simply making auto-fill-function interactive is not enough, since
auto-fill-function is a no-op.  The following redefinition of
auto-fill-function makes step 3 above DTRT, but I suspect it is not the
right fix, since I assume auto-fill-function was meant to be a no-op:

(defun auto-fill-function ()
  "Automatically break line at a previous space, in insertion of text."
  (interactive)
  (auto-fill-mode))

In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4)
 of 2009-09-17 on escher
Windowing system distributor `The X.Org Foundation', version 11.0.10502000
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil