Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

tests: Only log critical errors from scapy

Since 2.4.5 scapy started issuing warnings about a few different
configurations during our tests. These are harmless, but they generate
stderr output, which upsets atf_check.

Configure scapy to only log critical errors (and thus not warnings) to
fix these tests.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

Add basic IPv4/IPv6 forwarding tests.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24002