nixos: add Duo Security module

This module adds the security.duosec attributes, which you can use to
enable simple two-factor authentication for NixOS logins.

The module currently provides PAM and SSH support, although the PAM unix
system configuration isn't automatically dealt with (although the
configuration is automatically built).

Enabling it is as easy as saying:

  security.duosec.ssh.enable = true;
  security.duosec.ikey       = "XXXXXXXX...";
  security.duosec.skey       = "XXXXXXXX...";
  security.duosec.host       = "api-XXXXXXX.duosecurity.com";
  security.duosec.group      = "duosec";

which will enforce two-factor authentication for SSH logins for users in
the 'duosec' group.

This requires uid/gid support in the environment.etc module.

Signed-off-by: Austin Seipp <aseipp@pobox.com>