Add toggle_logo, make theme settings more usable

There is no provision for changing toggle_logo from the default.
The attached patch for system.module will add the setting for toggle_logo.

--- details ---
In the themes distributed with Drupal (4.7.0 rc1) the variable $logo is tested for inclusion of the logo. However, $logo is a string and always contains a path value; therefore it is always TRUE.

$logo is set by calling theme_get_setting('logo') in line 197 of the phptemplate.engine.

theme_get_setting() returns the value for 'logo' in this block starting at line 299 of theme.inc this block tests for $settings['toggle_logo'] which is set to a default setting of 1 in the function theme_get_settings().

    if ($settings['toggle_logo']) {
      $settings['logo'] = base_path();
      if ($settings['default_logo']) {
        $settings['logo'] .= dirname($theme_object->filename) .'/logo.png';
      }
      elseif ($settings['logo_path']) {
        $settings['logo'] .= $settings['logo_path'];
      }
    }

so toggle_logo is always TRUE and $logo will always have a path .....