chown(ref): parse_uid flattened syntax

[hlsxx]

chown: flatten parse_uid match arms and consolidate comments

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[cakebaker]

While it's an improvement, I'm not a fan of the Err(_)s. Why not flatten it even more by getting rid of the matches and doing the steps sequentially with early returns?