Issue #2854212: Rename Drupal to Backdrop (almost everywhere).

Some links to drupal.org still need to be drupal, for example, in
READEME.md.

audiofield.module

@@ -11,7 +11,7 @@ function audiofield_menu() {
   $items['admin/config/media/audiofield'] = array(
     'title' => 'Audio Field',
     'description' => 'Configure Audiofield.',
-    'page callback' => 'drupal_get_form',
+    'page callback' => 'backdrop_get_form',
     'page arguments' => array('audiofield_admin_settings_form'),
     'access arguments' => array('administer site configuration'),
     'type' => MENU_NORMAL_ITEM,
@@ -219,7 +219,7 @@ function audiofield_admin_settings_form_submit($form, &$form_state) {
   $config->set('audioplayer_ogg', $form_state['values']['audiofield_audioplayer_ogg']);
   $config->set('players_dir', $form_state['values']['audiofield_players_dir']);
   $config->save();
-  drupal_set_message(t('The AudioField configuration options have been saved.'));
+  backdrop_set_message(t('The AudioField configuration options have been saved.'));
 }
 
 /**

audiofield.players.inc

@@ -255,14 +255,14 @@ function audiofield_soundmanager2($player_path, $audio_file) {
       '#type' => 'markup',
       '#markup' => '<!--[if IE]><script type="text/javascript" src="' . $player_path . '/demo/360-player/script/excanvas.js"></script><![endif]-->',
     );
-    drupal_add_html_head($ie_js, 'audiofield');
+    backdrop_add_html_head($ie_js, 'audiofield');
 
-    drupal_add_js($player_base_path . '/script/soundmanager2.js', array('weight' => 0));
+    backdrop_add_js($player_base_path . '/script/soundmanager2.js', array('weight' => 0));
 
     //360 viewer
-    drupal_add_js($player_base_path . '/demo/360-player/script/berniecode-animator.js', array('weight' => 1));
-    drupal_add_js($player_base_path . '/demo/360-player/script/360player.js', array('weight' => 1));
-    drupal_add_css($player_base_path . '/demo/360-player/360player.css');
+    backdrop_add_js($player_base_path . '/demo/360-player/script/berniecode-animator.js', array('weight' => 1));
+    backdrop_add_js($player_base_path . '/demo/360-player/script/360player.js', array('weight' => 1));
+    backdrop_add_css($player_base_path . '/demo/360-player/360player.css');
 
     return '
     	<div id="sm2-container-real" class="ui360">