bug#56400: Make setopt warn on type mismatch

[Juri Linkov]

> Why not make `setopt' warn in case there is a type mismatch?
> For example, in:
>
>     (setopt tetris-buffer-width "hello")
>
> But the type of `tetris-buffer-width' is natnum.
>
> I think this would save users from shooting themselves in the foot.
> If they are sure they want to do that, they can use `setq' or
> `customize-set-variable' directly.

This is exactly what `set-variable' does.