Config form does not save page visibility negate option: Show for the listed pages

Hi,

I think I've tracked down this issue to how the radio form element is rendered. Also, this is the same UX for the block page visibility section. I've changed the priority to minor, as the default condition negate value is false, so the condition still evaluates correctly, However, it's rather poor UX.

In \Drupal\Core\Render\Element\Radio::preRenderRadio(), see

    if (isset($element['#return_value']) && $element['#value'] !== FALSE && $element['#value'] == $element['#return_value']) {
      $element['#attributes']['checked'] = 'checked';
    }

As the form option is false, this if statement is false and checked attribute is not set appropriately. Need to dig in to see if this is a core bug in radio rendering or in block visibility with how we manage this false option.

Here's a bit more info that I've tracked down related to the specific logic in the radio render.

- git history on 8.x branch does not reveal much about this logic.
- looks like this logic was added D7 theme_radio() when compared to D6 theme_radio(), see https://api.drupal.org/api/drupal/includes%21form.inc/function/theme_rad...
- logic was added here > https://www.drupal.org/node/971120#comment-3706168

Alright, so I've posted a drupal core bug report here > https://www.drupal.org/node/2450637.

Until that's picked up, I've committed a fix that shuffles the negate #default_option from a boolean to an integer as a temp work-around