Ruby master - Feature #16411: Safer keyword argument extension

I don't think adding **!? would be a good idea. I'd also ideally keep
the complexity in ruby less, when possible; it's (to me) interesting
to see that keywords add to the complexity. Ideally it should be so
simple for everyone to understand that there would never be any confusion
about it. IMO that was one rationale why it was changed for ruby 3.0.