dovecot: dovenull user should have its own group.

Quoting from https://wiki.dovecot.org/UserIds#dovenulluser:

"It should belong to its own private dovenull group where no one else
belongs to..."

nixos/modules/misc/ids.nix

@@ -385,7 +385,7 @@
       virtuoso = 44;
       #rtkit = 45; # unused
       dovecot2 = 46;
-      #dovenull = 47; # unused
+      dovenull2 = 47;
       prayer = 49;
       mpd = 50;
       clamav = 51;

nixos/modules/services/mail/dovecot.nix

@@ -311,7 +311,7 @@ in
       { name = "dovenull";
         uid = config.ids.uids.dovenull2;
         description = "Dovecot user for untrusted logins";
-        group = cfg.group;
+        group = "dovenull";
       }
     ] ++ optional (cfg.user == "dovecot2")
          { name = "dovecot2";
@@ -332,6 +332,10 @@ in
       }
     ++ optional (cfg.createMailUser && cfg.mailGroup != null)
       { name = cfg.mailGroup;
+      }
+    ++ singleton
+      { name = "dovenull";
+        gid = config.ids.gids.dovenull2;
       };
 
     environment.etc."dovecot/modules".source = modulesDir;