Improve WatchdogPruneSettings to better handle dblog_row_limit needing to be set to 0.

Problem/Motivation

Logic in WatchdogPruneSettings relating to dblog_row_limit needing to be set to 0 could be better handled by checking the actual setting in the system, currently it just appears hardcoded.

    $form['core_fs']['dblog_row_limit'] = [
      '#type' => 'select',
      '#title' => $this->t('[From Drupal Core:] Database log messages to keep'),
      '#options' => ['0' => 'All'],
      '#default_value' => 0,
      '#description' => $this->t('For this module to function, we must keep this Drupal Core setting set to <strong>All</strong>.  This setting is provided here simply as a reminder of where this setting is coming from.'),
    ];

Proposed resolution

Refactor WatchdogPruneSettings form, this might be a good time to actually refactor other some other settings, add better validation etc.