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/uploadcourse/classes/course.php

@@ -347,7 +347,7 @@ class tool_uploadcourse_course {
     /**
      * Get the directory of the object to restore.
      *
-     * @return string|false|null subdirectory in $CFG->tempdir/backup/..., false when an error occured
+     * @return string|false|null subdirectory in $CFG->backuptempdir/..., false when an error occured
      *                           and null when there is simply nothing.
      */
     protected function get_restore_content_dir() {