mailsystem: Add vmail user and group configuration

mailsystem/user.nix

@@ -0,0 +1,22 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  cfg = config.mailsystem;
+in {
+  config = lib.mkIf cfg.enable {
+    users.users."${cfg.vmailUserName}" = {
+      uid = cfg.vmailUID;
+      isSystemUser = true;
+      group = cfg.vmailGroupName;
+      home = cfg.mailDirectory;
+      createHome = true;
+      description = "Virtual Mail User";
+    };
+
+    users.groups."${cfg.vmailGroupName}" = {
+      gid = cfg.vmailUID;
+    };
+  };
+}