Use of ld_checkport in check_dns

All other checks use the sub ld_checkport to get the real server port
for the service checks.
So we unify the code that the check_dns use the same sub to avoid
the an special solution.

ldirectord/ldirectord.in

@@ -3637,6 +3637,7 @@ sub check_dns
 	my $request;
 	my $server;
 	my ($v,$r) = @_;
+	my $port = ld_checkport($v, $r);
 	{
 		# Net::DNS makes unguarded calls to eval
 		# which throw a fatal exception if they fail
@@ -3664,7 +3665,7 @@ sub check_dns
 		local $SIG{'ALRM'} = sub { die "timeout\n"; };
 		alarm($$v{negotiatetimeout});
 		$res->nameservers($server);
-		$res->port($$r{port}//$$v{port}//53);
+		$res->port($port);
 		if ($$v{"protocol"} eq "tcp") {
 			$res->usevc(1);
 		}