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

nixos/modules/services/mail/mail.nix

@@ -0,0 +1,33 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    services.mail = {
+
+      sendmailSetuidWrapper = mkOption {
+        default = null;
+        description = ''
+          Configuration for the sendmail setuid wrwapper (like an element of
+          security.setuidOwners)";
+        '';
+      };
+
+    };
+
+  };
+
+  ###### implementation
+
+  config = mkIf (config.services.mail.sendmailSetuidWrapper != null) {
+
+    security.setuidOwners = [ config.services.mail.sendmailSetuidWrapper ];
+
+  };
+
+}