Move all of NixOS to nixos/ in preparation of the repository merge

nixos/modules/security/pam_usb.nix

@@ -0,0 +1,41 @@
+{config, pkgs, ...}:
+
+with pkgs.lib;
+
+let
+
+  inherit (pkgs) pam_usb;
+
+  cfg = config.security.pam.usb;
+
+  anyUsbAuth = any (attrByPath ["usbAuth"] false) config.security.pam.services;
+
+in
+
+{
+  options = {
+
+    security.pam.usb = {
+      enable = mkOption {
+        default = false;
+        description = ''
+          Enable USB login for all login system unless the service disabled
+          it.  For more information, visit <link
+          xlink:href="http://pamusb.org/doc/quickstart#setting_up" />.
+        '';
+      };
+
+    };
+
+  };
+
+  config = mkIf (cfg.enable || anyUsbAuth) {
+
+    # pmount need to have a set-uid bit to make pam_usb works in user
+    # environment. (like su, sudo)
+
+    security.setuidPrograms = [ "pmount" "pumount" ];
+    environment.systemPackages = [ pkgs.pmount ];
+
+  };
+}