Lua loader: Add barebones "lfs" module

Add a Lua FileSystem module, an emulation of a subset of the permissively
licensed (MIT) Lua library of the same name[0], to our loader's Lua
environment.

[0]: https://github.com/keplerproject/luafilesystem/

Reviewed by: kevans
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14418