samples/landlock: Add support for named UNIX domain socket restrictions

samples/landlock: Add support for named UNIX domain socket restrictions

The access right for UNIX domain socket lookups is grouped with the
read-write rights in the sample tool.  Rationale: In the general case,
any operations are possible through a UNIX domain socket, including
data-mutating operations.

Cc: Justin Suess <utilityemal77@gmail.com>
Cc: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-7-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>