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/common.nix

@@ -16,6 +16,7 @@
 
 {
   config,
+  options,
   pkgs,
   lib,
 }:
@@ -86,4 +87,6 @@ in
       chmod 600 ${destination}
     '';
 
+  dovecotUnitName = if options.services.dovecot2 ? hasNewUnitName then "dovecot" else "dovecot2";
+
 }