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

nixos/modules/system/boot/shutdown.nix

@@ -0,0 +1,27 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+{
+
+  # This unit saves the value of the system clock to the hardware
+  # clock on shutdown.
+  systemd.units."save-hwclock.service" =
+    { wantedBy = [ "shutdown.target" ];
+
+      text =
+        ''
+          [Unit]
+          Description=Save Hardware Clock
+          DefaultDependencies=no
+          Before=shutdown.target
+
+          [Service]
+          Type=oneshot
+          ExecStart=${pkgs.utillinux}/sbin/hwclock --systohc ${if config.time.hardwareClockInLocalTime then "--localtime" else "--utc"}
+        '';
+    };
+
+  boot.kernel.sysctl."kernel.poweroff_cmd" = "${config.systemd.package}/sbin/poweroff";
+
+}