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

[martin rudalics]

>  > I also tried setting an explicit `nil` value and got an error, but
>  > judging by how `internal-border-width` shows the same behaviour I assume
>  > that it's working as expected.
>
> Indeed.  That's why I dislike frame parameters.  WOW users cannot set
> 'internal-border-width' back to its initial value once it changed.  For
> scroll bars we use a two-tiers approach where users can tweak width and
> type separately.  Doing something similar for the internal border seems
> a bit excessive to me.

I think I fixed that now in the sense that an explicit nil value does
not get an error but triggers the fallback mechanism.  Please check out
the latest master.

Thanks, martin