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