landlock: Document LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET

Introduce LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET as an IPC scoping
mechanism available since Landlock ABI version 6. Update ruleset_attr,
Landlock ABI version, and access rights code blocks based on that.

Signed-off-by: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/ac75151861724c19ed62b500cfe497612d9a6607.1725494372.git.fahimitahera@gmail.com


[mic: Improve commit message and documentation, add a missing
fallthrough, reformat to 80 columns, improve some wording]
Signed-off-by: Mickaël Salaün <mic@digikod.net>