Remove $FreeBSD$: one-line sh pattern

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

Switch rtsock tests to per-test jails and epair interfaces.

Many rtsock tests verify the ordering of the kernel messages for the
particular event. In order to avoid flaky tests due to the other tests
running, switch all tests to use personal vnet-enabled jails.
This removes all clashes on the IP addresses and brings back the ability
to run these tests simultaneously.

Reported by: olivier
Reviewed by: olivier
Differential Revision: https://reviews.freebsd.org/D24182