bug#5294: 23.1; unload-feature disable minor-mode

[Kevin Ryde]

As an idea for unload-feature, when unloading a buffer-local minor mode
it could helpfully find buffers where the mode is enabled and disable it
before unloading.

An example foo.el mode below.  Eval the code in try-foo.el and it leaves
the buffer boldened, where disabling the mode could have undone it.

Of course foo.el can do something like the commented-out
`foo-unload-function' itself, but I think almost all minor modes would
benefit from this and that `unload-feature' might therefore handle it.

Identifying a minor mode function would be as easy as looking in
`minor-mode-list' would it?  Otherwise I expect define-minor-mode could
chuck some code in `foo-unload-hook' - if it presumes the feature symbol
will match the load filename.

foo.el
Description: application/emacs-lisp

try-foo.el
Description: application/emacs-lisp

In GNU Emacs 23.1.1 (i486-pc-linux-gnu, GTK+ Version 2.16.5)
 of 2009-09-14 on raven, modified by Debian
configured using `configure  '--build=i486-linux-gnu' '--host=i486-linux-gnu' 
'--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' 
'--localstatedir=/var/lib' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' 
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.1/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.1/leim'
 '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 
'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g 
-O2' 'LDFLAGS=-g' 'CPPFLAGS=''