bug#47352: Numbered backups also need kept-new-versions else will grow out of control

[Dan Jacobson]

(info "(coreutils) Backup options") has "numbered",
but without a corresponding limit factor, these can grow beyond belief.

So coreutils also needs a way to say like emacs does:

kept-new-versions is a variable defined in ‘files.el’.
Documentation:
Number of newest versions to keep when a new numbered backup is made.
Includes the new backup.  Must be greater than 0.