Use file_transfer() as a helper function instead of inlining file transfers

Based on the discussion around file_transfer(): #1463656: Add a Drupal kernel; leverage HttpFoundation and HttpKernel.

8.x: file_transfer() is used by image_style_deliver() and file_download().

The kernel patch introduces StreamedResponses there to transfer files, inlining the code for streaming the files.

Proposed resolution

• Make file_transfer() a helper function that returns a StreamedResponse.
• Revert changes to core/includes/file.inc:
  

  diff --git a/core/includes/file.inc b/core/includes/file.inc
  index f204989..91e7eca 100644
  --- a/core/includes/file.inc
  +++ b/core/includes/file.incundefined
  @@ -5,6 +5,9 @@
    * API for handling file uploads and server file management.
    */
   
  +use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
  +use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
  +use Symfony\Component\HttpFoundation\StreamedResponse;
   use Drupal\Core\StreamWrapper\LocalStream;
   
   /**
  @@ -2026,18 +2029,27 @@ function file_download() {
         $function = $module . '_file_download';
         $result = $function($uri);
         if ($result == -1) {
  -        return drupal_access_denied();
  +        throw new AccessDeniedHttpException();
         }
         if (isset($result) && is_array($result)) {
           $headers = array_merge($headers, $result);
         }
       }
       if (count($headers)) {
  -      file_transfer($uri, $headers);
  +      return new StreamedResponse(function() use ($uri) {
  +        $scheme = file_uri_scheme($uri);
  +        // Transfer file in 1024 byte chunks to save memory usage.
  +        if ($scheme && file_stream_wrapper_valid_scheme($scheme) && $fd = fopen($uri, 'rb')) {
  +          while (!feof($fd)) {
  +            print fread($fd, 1024);
  +          }
  +          fclose($fd);
  +        }
  +      }, 200, $headers);
       }
  -    return drupal_access_denied();
  +    throw new AccessDeniedHttpException();
     }
  -  return drupal_not_found();
  +  throw new NotFoundHttpException();
   }

• Use the new file_transfer() in core/modules/update/tests/modules/update_test/update_test.module, instead of creating a custom StreamedResponse. Or is it for managed files only?
  

  -  readfile("$path/$project_name.$availability_scenario.xml");
  +  $file = "$path/$project_name.$availability_scenario.xml";
  +  if (!is_file($file)) {
  +    // Return an empty response.
  +    return new Response('', 200, array('Content-Type' =>  'text/xml; charset=utf-8'));
  +  }
  +  return new StreamedResponse(function() use ($file) {
  +    // Transfer file in 1024 byte chunks to save memory usage.
  +    if ($fd = fopen($file, 'rb')) {
  +      while (!feof($fd)) {
  +        print fread($fd, 1024);
  +      }
  +      fclose($fd);
  +    }
  +  }, 200, array('Content-Type' =>  'text/xml; charset=utf-8'));

Also note #1561362: Change file_transfer() to use BinaryFileResponse that is about making a StreamedFileResponse or another replacement, or something in Symfony upstream.