samples/landlock: Add a sandbox manager example

Add a basic sandbox tool to launch a command which can only access a
list of file hierarchies in a read-only or read-write way.

Cc: James Morris <jmorris@namei.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210422154123.13086-12-mic@digikod.net
Signed-off-by: James Morris <jamorris@linux.microsoft.com>