dovecot: use marker option as unit name migration indicator

In nixpkgs we expose `services.dovecot.hasNewUnitName` option that can be
safely inspected to understand that whether to use the `dovecot` systemd
service name instead of `dovecot2`.

mail-server/users.nix

@@ -16,11 +16,21 @@
 
 {
   config,
+  options,
   pkgs,
   lib,
   ...
 }:
 
+with (import ./common.nix {
+  inherit
+    config
+    options
+    lib
+    pkgs
+    ;
+});
+
 with config.mailserver;
 
 let
@@ -107,7 +117,7 @@ in
 
     systemd.services.activate-virtual-mail-users = {
       wantedBy = [ "multi-user.target" ];
-      before = [ "dovecot2.service" ];
+      before = [ "${dovecotUnitName}.service" ];
       serviceConfig = {
         ExecStart = virtualMailUsersActivationScript;
       };