acl(3): improve discoverability of acl_get_perm_np(3)

• Mention it in acl(3) as an available function, xref
• Mention it in acl_get_permset(3), as acl_get_perm_np(3) is a natural follow-up to acl_get_permset(3)

Sponsored by: Klara, Inc.

(cherry picked from commit 20c61a7c518b9c2f451118913e10a1525ad0b578)