[$] Conventions for extensible system calls

The kernel does not have just one system call to rename a file; instead,there are three of them: rename(),renameat(), and renameat2(). Each was added when theprevious one proved unable to support a new feature. A similar story hasplayed out with a number of system calls: a feature is needed that doesn'tfit into the existing interfaces, so a new one is created - again. At the2020 Linux Plumbers Conference,Christian Brauner and Aleksa Sarai ran a pair of sessions focused on thecreation of future-proof system calls that can be extended when the needfor new features arises.