bug#24594: 24.5; `variable-pitch-mode': accept FACE arg instead of hardcoding the face

[Drew Adams]

> We already have buffer-face-mode.

Not really the same.

> > Was there some particular reason that a mode was written specifically for
> variable-pitch (whether the face `variable-pitch' or variable-pitch faces in
> general), rather than providing a mode for any face?  If not then agreed: a
> name change would be appropriate.
> 
> The variable-pitch face is a very general face: it stands for a face
> using any variable-pitch font, of which there are gazillions.

A single face.  The face is hard-coded in the command.  You cannot
use the command with a different face.  Being able to customize
a face is something else altogether.

And no, there is nothing special about face `variable-pitch'.  In
particular, there is nothing that prevents you from customizing it
to a fixed-pitch face.