a callback function instead of an array of options.
* Admin setting admin_setting_configselect now supports validating the selection by supplying a
callback function.
+* The task system has new functions adhoc_task_starting() and scheduled_task_starting() which must
+ be called before executing a task, and a new function \core\task\manager::get_running_tasks()
+ returns information about currently-running tasks.
+ * New library function rename_to_unused_name() to rename a file within its current location.
=== 3.9 ===
* Following function has been deprecated, please use \core\task\manager::run_from_cli().
