always put Customizations in `custom-file', never in `user-init-file'

[Drew Adams]

Having customizations go into `custom-file' has a great advantage over
having them stuck in `user-init-file' (.emacs): The entire `custom-file' is
generally not intended to be edited by a user, whereas `user-init-file' is
specifically intended to be edited.

I think it was probably a mistake ever to have put Customize customizations
into a file intended for editing. That can confuse users, and it's just
asking for trouble. Now that we have a good alternative (`custom-file'), I
think that should be used exclusively.

nil should then not be allowed as the value of `custom-file'. The default
value could be something like `~/.emacs-custom-file'.

You might say that this would be too disruptive, because many people already
have Customizations in `user-init-file'.

To handle that, Emacs would move any existing Customizations from
`user-init-file' to `custom-file' (replacing them at that place by a load of
`custom-file'). This would be done when `user-init-file' is loaded if
Customizations are encountered there. The user would be informed when this
happens. Customize could even be opened to `custom-file' at that time, to
let the user specify the `custom-file' to use.

A user would not be able to set `custom-file' to the value of
`user-init-file' or vice versa: an error would be raised whenever either is
attempted.

WDOT?