net/mgen: Update 4.2b6 => 5.1.1, take maintainership

Changelog:
https://github.com/USNavalResearchLaboratory/mgen/blob/915e88a/VERSION.TXT

Project moved to:
https://www.nrl.navy.mil/Our-Work/Areas-of-Research/Information-Technology/NCS/MGEN/
https://github.com/USNavalResearchLaboratory/mgen/

Improve port:

• Unbreak build on 64bit ARCHes.
• Add LICENSE.
• Fix warnings from portclippy.
• Optimize install DOCS.

PR: 295246
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q2

(cherry picked from commit 6b5823b06a44a5f9eca85b81860271345682336d)