flua: add fbsd module

[bapt]

Summary

This module is bundled into flua, it only provides for now the exec
function. The point of the function is to be able to execute a program
without actually executing a shell.

to use it:
fbsd.exec("id", "bapt")

[manu]

libexec/flua/modules/lfbsd.c

75

leftover from porting this from pkg :)

[bapt]

address @manu's comment