gateways: far gateway IPv6 for https://github.com/opnsense/core/issues/4436

(I'm not sure this works, let's ask for a test first)

src/www/system_gateways_edit.php

@@ -129,7 +129,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                         }
                     }
 
-                    if (!$found) {
+                    if (!$found && !isset($pconfig['fargw'])) {
                         $input_errors[] = sprintf(gettext('The gateway address "%s" does not lie within one of the chosen interface\'s IPv6 subnets.'), $pconfig['gateway']);
                     }
                 }