time-wait-sync: use watchfile to coordinate with timesyncd

time-wait-sync: use watchfile to coordinate with timesyncd

Systems that have an accurate real-time clock may have an initial
unsynchronized time that is close enough to the synchronized time that
the final adjustment doesn't trigger a waking "clock set" event.  Have
timesyncd touch a file in its runtime directory as a secondary signal
for synchronization.  Continue to support the timerfd-based trigger as a
sufficient condition when the watchfile is not present.

Closes issue #8683