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/dovecot.nix

@@ -16,22 +16,24 @@
 
 {
   config,
+  options,
   pkgs,
   lib,
   ...
 }:
 
-with (import ./common.nix { inherit config pkgs lib; });
+with (import ./common.nix {
+  inherit
+    config
+    options
+    pkgs
+    lib
+    ;
+});
 
 let
   cfg = config.mailserver;
 
-  dovecotPreStart =
-    ''
-      ${genPasswdScript}
-    ''
-    + (lib.optionalString cfg.ldap.enable setPwdInLdapConfFile);
-
   passwdDir = "/run/dovecot2";
   passwdFile = "${passwdDir}/passwd";
   userdbFile = "${passwdDir}/userdb";
@@ -445,10 +447,13 @@ in
       '';
     };
 
-    systemd.services.dovecot.preStart = lib.mkIf (config.systemd.services ? "dovecot") dovecotPreStart;
-    systemd.services.dovecot2.preStart = lib.mkIf (
-      config.systemd.services ? "dovecot2"
-    ) dovecotPreStart;
+    systemd.services.${dovecotUnitName} = {
+      preStart =
+        ''
+          ${genPasswdScript}
+        ''
+        + (lib.optionalString cfg.ldap.enable setPwdInLdapConfFile);
+    };
 
     systemd.services.postfix.restartTriggers = [
       genPasswdScript