Now that we know if we're shutting down, enable locking in resettodr().
In the shutting_down case, we acquire the todr_mutex as a trylock.  Under
most circumstances, this lock will be uncontended.  But if we happen to
panic while accessing the TOD clock (via clock_settime() or whatever),
then trylock will prevent us from getting stuck (and a warning will be
displayed on the console).

(thorpej)