Group storage and vmail user options at mailserver.storage

Create a nicer option structure that deals with the mail storage and its
owner, uid, group and gid. Also includes the directory layout as a
property of how mails are stored..

mail-server/users.nix

@@ -34,15 +34,6 @@ with (import ./common.nix {
 let
   cfg = config.mailserver;
 
-  vmail_user = {
-    name = cfg.vmailUserName;
-    isSystemUser = true;
-    uid = cfg.vmailUID;
-    home = cfg.mailDirectory;
-    createHome = true;
-    group = cfg.vmailGroupName;
-  };
-
   virtualMailUsersActivationScript =
     pkgs.writeScript "activate-virtual-mail-users"
       # bash
@@ -57,7 +48,7 @@ let
         # Create directory to store user sieve scripts if it doesn't exist
         if (! test -d "${cfg.sieveDirectory}"); then
           mkdir "${cfg.sieveDirectory}"
-          chown "${cfg.vmailUserName}:${cfg.vmailGroupName}" "${cfg.sieveDirectory}"
+          chown "${cfg.storage.owner}:${cfg.storage.group}" "${cfg.sieveDirectory}"
           chmod 770 "${cfg.sieveDirectory}"
         fi
 
@@ -69,13 +60,13 @@ let
             ''
               if (! test -d "${cfg.sieveDirectory}/${name}"); then
                 mkdir -p "${cfg.sieveDirectory}/${name}"
-                chown "${cfg.vmailUserName}:${cfg.vmailGroupName}" "${cfg.sieveDirectory}/${name}"
+                chown "${cfg.storage.owner}:${cfg.storage.group}" "${cfg.sieveDirectory}/${name}"
                 chmod 770 "${cfg.sieveDirectory}/${name}"
               fi
               cat << 'EOF' > "${cfg.sieveDirectory}/${name}/default.sieve"
               ${sieveScript}
               EOF
-              chown "${cfg.vmailUserName}:${cfg.vmailGroupName}" "${cfg.sieveDirectory}/${name}/default.sieve"
+              chown "${cfg.storage.owner}:${cfg.storage.group}" "${cfg.sieveDirectory}/${name}/default.sieve"
             ''
           else
             ''
@@ -113,16 +104,18 @@ in
           )
         );
 
-    # set the vmail gid to a specific value
-    users.groups = {
-      "${cfg.vmailGroupName}" = {
-        gid = cfg.vmailUID;
-      };
+    users.groups.${cfg.storage.group} = {
+      inherit (cfg.storage) gid;
     };
-
-    # define all users
-    users.users = {
-      "${vmail_user.name}" = lib.mkForce vmail_user;
+    users.users.${cfg.storage.owner} = lib.mkForce {
+      inherit (cfg.storage)
+        group
+        uid
+        ;
+      name = cfg.storage.owner;
+      isSystemUser = true;
+      home = cfg.storage.path;
+      createHome = true;
     };
 
     systemd.services.activate-virtual-mail-users = {