nixos: fix linux containers (systemd-nspawn, lxc, lxc-libvirt)

- Make dhcp work, use dhcpcd without udev in container
- Make login shell work, patch getty to not wait for /dev/tty0
- Make ssh work, sshd/pam do not start session
Merge branch 'nixos/containers/fix1' of git://github.com/offlinehacker/nixpkgs

nixos: fix linux containers (systemd-nspawn, lxc, lxc-libvirt)