MFC r364449, r365004: ls(1): Update POSIX conformance

• Update the POSIX references for non-standard ls(1) options
• Simplify the STANDARDS section by mention both supported POSIX versions

Verified by checking [1].

[1] https://pubs.opengroup.org/onlinepubs/9699919799.2016edition/toc.htm

PR: 140435 (based on)
Submitted by: Dan Lukes <dan at obluda dot cz>
Reviewed by: hrs, bcr
Approved by: hrs, bcr
Differential Revision: https://reviews.freebsd.org/D26210