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

[Drew Adams]

> > 2. Enhancement: Fix face `variable-pitch' so that it really
> > limits customization of the attributes to variable-pitch
> > faces.  That is the only justification for a face name
> > that reflects particular attribute values: limit
> > customization for those values.
> 
> Hmm, it looks like there is no way from lisp to tell if a face (or
> font) is variable-pitch or not. Could be something useful to have in
> and of itself.

I was thinking the same thing, but was unsure how this might
be done (e.g., whether it is possible now).

> > Again, the only reason to keep `variable-pitch-mode' and
> > face `variable-pitch' is if you feel that we really need
> > such things.  (I do not.)
> 
> Before deciding if something is really needed/wanted, we would have to
> see what people use the feature for. You can't provide that
> information since you don't use it.

Agreed.  Which is why I proposed to improve it, unless
someone (else) can decide that it should just be removed.