sched: add function to remove all timers and descriptors

This allows a helper process to be started in an *_Initialise() call
and use the scheduler (unlike the privops helper, which has its own
loop).

sched.c

@@ -472,6 +472,20 @@ SCH_RemoveTimeout(SCH_TimeoutID id)
   assert(0);
 }
 
+/* ================================================== */
+
+void
+SCH_Reset(void)
+{
+  while (n_timer_queue_entries > 0)
+    SCH_RemoveTimeout(timer_queue.next->id);
+
+  while (one_highest_fd > 0) {
+    close(one_highest_fd - 1);
+    SCH_RemoveFileHandler(one_highest_fd - 1);
+  }
+}
+
 /* ================================================== */
 /* Try to dispatch any timeouts that have already gone by, and
    keep going until all are done.  (The earlier ones may take so