bug#32643: 26; minor-mode variables

[Drew Adams <drew.adams@oracle.com>]

>   > Applying common sense, the obvious explanation is that auto-fill-mode
>   > controls only a single variable (i.e., `auto-fill-function'), and adding
>   > an extra `auto-fill-mode' which would then have to be kept in sync just
>   > to conform to some convention is not worth the trouble.
> 
> We could arrange for Auto-Fill mode to be controlled by a variable
> auto-fill-mode.  Change the auto-fill code so that two variables
> control it: (and auto-fill-mode auto-fill-function (funcall
> auto-fill-function ...)).  Then we could leave auto-fill-function set
> up all the time, and enable/disable the mode by setting auto-fill-mode
> as with most other minor modes.

I do hope that someone does something, perhaps along those lines,
to provide an `auto-fill-mode' variable.

As I said, I'm not familiar with the `:variable' stuff. I'd think that using
`:set' would be sufficient, to ensure that both variables are updated
correctly whenever `auto-fill-mode' is set. But perhaps that would
not suffice to ensure that whenever `auto-fill-function' is set.

(A guess is that using `:variable' provides a simpler or more elegant
way of doing what could be done using `:set'.)