flua: Add a freebsd module

[markj]

Summary

This provides an example of FreeBSD-specific functionality that does not
belong in the posix module. Specifically, this wraps a couple of the
easy capsicum system calls. To do the rest, we need to make use of a
file descriptor userdata.

As with the previous diff, this isn't really committable yet.

[bapt]

I think we should not bundle it in flua but have it dynamically loaded, what do you think? I have an exemple https://reviews.freebsd.org/D46558