Some fixes to work with the current NixOS again

The File::Slurp dependency was added in https://github.com/NixOS/nixpkgs/commit/7c480ad89695c1309cc2ed5d07c166722476cd8c

I'm not sure I understand why the path to mkdir needed to be explicitly spelled out in openssh.nix, but it was failing without it.

Tested on NixOS 14.11pre54200.0b1d16c