Replace nixpkgs overlays with a nixpkgs fork and upgrade to OpenSSL 1.1.1k

Fixes #62 (closed)

Supersedes !69 (closed)

The changes this introduces are visible at https://github.com/PrivateStorageio/nixpkgs/compare/nixos-19.09...PrivateStorageio:31c7a57bd9972979275fbfd9e490bfb502154c11