Network management UI: add system proxy default TUN Mode

https://man.freebsd.org/cgi/man.cgi?query=tun
Network management UI added option TUN
TUN acts as an agent for existing agreements, HTTP proxy HTTPS proxy SOCKS proxy
Use TUN mode to proxy all traffic throughout the system, including terminals

Why this request?

I’m in a country where the internet is very blocked, and upgrading or downloading software is a hassle, github gitlab Ping over time 300ms.

In thousands of Linux and BSD distributions, there is no single system desktop environment that proxies all traffic,
I use a proxychains and it’s a terrible proxy for the terminal, very slow.