landlock

Extend the help with the latest Landlock ABI version supported by the
sandboxer.

Inform users about the sandboxer or the kernel not being up-to-date.

Make the version check code easier to update and harder to misuse.

Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20220923154207.3311629-2-mic@digikod.net