samples/landlock: Refactor help message

Help message is getting larger with each new supported feature (scopes,
and soon UDP). Also the large number of calls to fprintf with
environment variables make it hard to read. Refactor it away into a
single simpler constant format string.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://lore.kernel.org/r/20241019151534.1400605-3-matthieu@buffet.re


[mic: Move the small cleanups in the next commit]
Signed-off-by: Mickaël Salaün <mic@digikod.net>