Add Nix, a purely functional package manager

Nix is a purely functional package manager. This means that it treats
packages like values in purely functional programming languages such as
Haskell -- they are built by functions that don't have side-effects,
and they never change after they have been built.

FreeBSD support in Nix is not fully complete yet. This commit only
brings the Nix package manager to the ports. Hopefully, this port will
streamline the work of bootstrapping of Nix packages on FreeBSD.

Thanks to all the kind folks who contributed to the porting efforts.
It was a fun journey.

WWW: https://nixos.org/nix/

Differential Revision: https://reviews.freebsd.org/D17766