Move 'default_nodes_main' setting to node module

Moving 'default_nodes_main' to a separate node_form_system_site_information_settings_alter() will change a form structure because it makes no sense to have a fieldset with only one 'site_frontpage' field in fieldset. Also this setting used in taxonomy pages so we could have other variable for them

$form['front_page'] = array(
    '#type' => 'fieldset', 
    '#title' => t('Front page'),
  );
  $form['front_page']['default_nodes_main'] = array(
    '#type' => 'select', 
    '#title' => t('Number of posts on front page'), 
    '#default_value' => variable_get('default_nodes_main', 10), 
    '#options' => drupal_map_assoc(array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30)), 
    '#description' => t('The maximum number of posts displayed on overview pages such as the front page.'),
  );
  $form['front_page']['site_frontpage'] = array(

Proposed

  $form['site_frontpage'] = array(