bug#46184: 28.0.50; child-frame-border-width of 0 falls back to internla-border-width

[martin rudalics]

> (make-frame `(
>          (parent-frame . ,(window-frame))
>          (child-frame-border-width . 0)
>          (internal-border-width . 200)))
>
> I would expect the border width to only fall back if
> child-frame-border-width is nil, since 0 is a useful setting.

It's not entirely trivial to do that (frame parameters are a pain in
this regard) but please have a look at the attached patch (tested on a
GTK build only).  Alexander, please have a look too.

Thanks, martin

child-frame-border-width.diff
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