restructuring; now works with 17.03/17.09-pre

I had to remove some of the features, they need to be slowely added in
again.

mail-server/users.nix

@@ -0,0 +1,27 @@
+#  nixos-mailserver: a simple mail server
+#  Copyright (C) 2016-2017  Robin Raymond
+#
+#  This program is free software: you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation, either version 3 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program. If not, see <http://www.gnu.org/licenses/>
+
+{ vmail_id_start, vmail_user, mail_user }:
+
+{
+  # set the vmail gid to a specific value
+  users.groups = {
+    vmail = { gid = vmail_id_start; };
+  };
+
+  # define all users
+  users.extraUsers = vmail_user ++ mail_user;
+}