Remove $FreeBSD$: one-line sh pattern

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

flua: Add a libjail module

libjail is pretty small, so it makes for a good proof of concept demonstrating
how a system library can be wrapped to create a loadable Lua module for flua.

* Introduce 3lua section for man pages
* Add libjail module

Reviewed by: kevans, manpages
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D26080