MDL-60923 backup: Added $CFG->backuptempdir

The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.

Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.

admin/tool/recyclebin/classes/course_bin.php

@@ -211,9 +211,9 @@ class course_bin extends base_bin {
         // Get the backup file.
         $file = reset($files);
 
-        // Get a temp directory name and create it.
+        // Get a backup temp directory name and create it.
         $tempdir = \restore_controller::get_tempdir_name($context->id, $user->id);
-        $fulltempdir = make_temp_directory('/backup/' . $tempdir);
+        $fulltempdir = make_backup_temp_directory($tempdir);
 
         // Extract the backup to tempdir.
         $fb = get_file_packer('application/vnd.moodle.backup');