bug#52543: Add option to eliminate all duplications from the kill ring

[ndame]

There is  kill-do-not-save-duplicates, but it deletes
only subsequent duplicates.

There should be an other variable which prevents any duplicates
even if there are, say,  10 or 20 items between them.

I expected kill-do-not-save-duplicates to do that, that variable
seems misnamed. It should be kill-do-not-save-subsequent-duplicates
instead.