adduser.8: update log location + spdx

Closes: 280538
Fixes: cf8a18 (back out logging to /var/log/adduser)
MFC after: 3 days
Reported by: Herbert Baerschneider <herbert.baerschneider@protonmail.com>

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1354

(cherry picked from commit 1f882a5834836b43d4689efeefbc3109fc169363)