Updating the Networking Section of the NetBSD Guide

[nia <nia%NetBSD.org@localhost>]

Hello, I'm doing my best to bring the "Networking in practice" section of
the guide into the current decade, since the guide is very outdated and
focused on Dial Up and IPF rather than NPF.

http://netbsd.org/docs/guide/en/chap-net-practice.html

I'm not a networking expert and I only know enough to configure a basic
SoHo router so I'd appreciate any advice and suggestions of things to add.