Sysusers lint does not meet specification

[M-Pixel]

bootc lint fails in ways that systemd itself does not when run on a system with files that contain certain features (here is the systemd specification).

• fails to parse when a /file/owned/by/user (or group) path is provided instead of a number or dash.
• ignores lines that begin with u!, instead of treating the same as u
• fails to parse u lines that leave GECOS field unset instead of explicitly defaulting it with -

[cgwalters]

Thanks for the report; PR in #1332