Improve password handling

[peti]

Instructions how to set-up passwords in a secure manner have been posted at http://permalink.gmane.org/gmane.linux.distributions.nixos/20368.

[splack]

That link is dead. Do you have a new one?

[peti]

https://mailman.science.uu.nl/pipermail/nix-dev/2016-May/020447.html