dovecot: add compat shim for dovecot unit name migration

In nixpkgs I renamed dovecot2 to dovecot and made dovecot2 an alias, so
adding the script to the alias does us no good.

mail-server/dovecot.nix

@@ -16,6 +16,7 @@
 
 {
   config,
+  options,
   pkgs,
   lib,
   ...
@@ -26,6 +27,8 @@ with (import ./common.nix { inherit config pkgs lib; });
 let
   cfg = config.mailserver;
 
+  dovecotUnitName = if lib.hasAttr "dovecot" options.systemd.services then "dovecot" else "dovecot2";
+
   passwdDir = "/run/dovecot2";
   passwdFile = "${passwdDir}/passwd";
   userdbFile = "${passwdDir}/userdb";
@@ -439,13 +442,11 @@ in
       '';
     };
 
-    systemd.services.dovecot2 = {
+    systemd.services.${dovecotUnitName}.preStart =
-      preStart =
+      ''
-        ''
+        ${genPasswdScript}
-          ${genPasswdScript}
+      ''
-        ''
+      + (lib.optionalString cfg.ldap.enable setPwdInLdapConfFile);
-        + (lib.optionalString cfg.ldap.enable setPwdInLdapConfFile);
-    };
 
     systemd.services.postfix.restartTriggers = [
       genPasswdScript