This is a forward-port of Andrea's fix in 2.4.

If a timer handler re-adds a timer to go off right now, __run_timers() will
never terminate.  (I wrote a test.  It happens.)

Fix that up by teaching internal_add_timer() to detect when it is being
called from within the context of __run_timers() and to park newly-added
timers onto a temp list instead.  These timers are then added for real by
__run_timers(), after it has finished processing all pending timers.