bug#53894: 27.2; Clipboard copy -> C-y -> M-y -> Same clipboard copy does not push to kill ring

[Ignacio Casso]

>> You are right that it is a dangerous assumption, and it's also hard to
>> test and probably unnecessary. So I have changed the code to check the
>> window-system variable instead and only use timestamps for X and haiku
>
> Haiku doesn't support timestamps, only `gui-backend-selection-owner-p'
> is reliable.  But I can expose the selection counter if you think that's
> a good idea.
>
> Thanks.

I would keep it simple for now and restrict this patch to things that
work out of the box right now. If you like the rest of the patch I can
remove haiku from the list and add any other window system that you tell
me.

Adapting the new code to work also with counters is straightforward
however. So if you think it's worth it to expose counters just to fix this
bug in haiku and other window systems, we can do it.