sysutils/symon: Improve port, take maintainership

• Convert to modern OPTIONS.
• Add record to UPDATING about options.
• "Samplefy" configs.
• Fix warnings from portclippy.
• "Unbreak" on 15+ - disable PF support (patches gratefully received!).

PR: 292366
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>