flua: Add hash module

Add lua bindings to hashing functions. sha256 is available. sha256.new
craetes a new object. sha256.update updates the digest. sha256.digest
returns the digest as a binary string and resets the
context. sha256.hexdigest returns the digest as a string of hex digits
and then resets the cotnext.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43872

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