Don't refuse to serve files from inside dataroot when dataroot has a symlink...

Don't refuse to serve files from inside dataroot when dataroot has a symlink in it (Eduforge tracker, #3324)
Signed-off-by: Richard Mansfield <richardm@catalyst.net.nz>

htdocs/lib/file.php

@@ -51,7 +51,7 @@ define('BYTESERVING_BOUNDARY', 'm1i2k3e40516'); //unique string constant
  *                         there are none.
  */
 function serve_file($path, $filename, $mimetype, $options=array()) {
-    $dataroot = get_config('dataroot');
+    $dataroot = realpath(get_config('dataroot'));
     $path = realpath($path);
     $options = array_merge(array(
         'lifetime' => 86400