Loosen the type of SSH key files

(cherry picked from commit 444a4fb793c67243512a7f17c287e5665a8e2a68)
Mark the value for nixpkgs.system as a default

(cherry picked from commit 9668294eeb57aaeb8220409e154b411263deedd8)
Don't set an initial null root password for Amazon / VirtualBox images

A null password allows logging into local PAM services such as "login"
(agetty) and KDM.  That's not actually a security problem for EC2
machines, since they do not have "local" logins; for VirtualBox
machines, if you local access, you can do anything anyway.  But it's
better to be on the safe side and disable password-based logins for
root.

(cherry picked from commit 4ba7dfde5b79ec835e8739922400b3f5f4f089f2)
Print a reminder to users to set or disable the root password

(cherry picked from commit 20093751ec96b43d87f0850fc3ad541237405564)
Include the NixOS version in the manual

In particular, this will make it clear what version of the manual is
shown at http://nixos.org/nixos/manual/.

(cherry picked from commit 9ef07d859b23840e1a5d7d7125a6982720afa391)