allow Home Manager to be used as a NixOS module

This is a NixOS module that is intended to be imported into a NixOS
system configuration. It allows the system users to be set up directly
from the system configuration.

The actual profile switch is performed by a oneshot systemd unit per
configured user that acts much like the regular `home-manager switch`
command.

With this implementation, the NixOS module does not work properly with
the `nixos-rebuild build-vm` command. This can be solved by using the
`users.users.<name?>.packages` option to install packages but this
does not work flawlessly with certain Nixpkgs packages. In particular,
for programs using the Qt libraries.

nixos/default.nix

@@ -0,0 +1,57 @@
+{ config, lib, pkgs, utils, ... }:
+
+with lib;
+
+let
+
+  cfg = config.home-manager;
+
+  hmModule = types.submodule ({name, ...}: {
+    imports = import ../modules/modules.nix {
+      inherit lib pkgs;
+      nixosSubmodule = true;
+    };
+
+    config = {
+      home.username = config.users.users.${name}.name;
+      home.homeDirectory = config.users.users.${name}.home;
+    };
+  });
+
+in
+
+{
+  options = {
+    home-manager.users = mkOption {
+      type = types.attrsOf hmModule;
+      default = {};
+      description = ''
+        Per-user Home Manager configuration.
+      '';
+    };
+  };
+
+  config = mkIf (cfg.users != {}) {
+    systemd.services = mapAttrs' (username: usercfg:
+      nameValuePair ("home-manager-${utils.escapeSystemdPath username}") {
+        description = "Home Manager environment for ${username}";
+        wantedBy = [ "multi-user.target" ];
+
+        serviceConfig = {
+          User = username;
+          Type = "oneshot";
+          RemainAfterExit = "yes";
+          SyslogIdentifier = "hm-activate-${username}";
+
+          # The activation script is run by a login shell to make sure
+          # that the user is given a sane Nix environment.
+          ExecStart = pkgs.writeScript "activate-${username}" ''
+            #! ${pkgs.stdenv.shell} -el
+            echo Activating home-manager configuration for ${username}
+            exec ${usercfg.home.activationPackage}/activate
+          '';
+        };
+      }
+    ) cfg.users;
+  };
+}