bug#24023: 24.5; Use `custom-file' (if defined), not init file, for `en/disable-command'

[Eli Zaretskii]

> Date: Mon, 18 Jul 2016 20:55:07 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> This very old feature still writes to your init file.  If you have
> defined `custom-file' then it should write to that file instead.

This feature is not a defcustom, so it doesn't work like one.  Perhaps
the easiest way forward would be to make it a defcustom, or something
like one.