time-sync.target is supposed to indicate system clock is synchronized
with a remote clock, but as used through 241 it only provided a system
clock that was updated based on a locally-maintained timestamp. Systems
that are powered off for extended periods would not come up with
accurate time.
Retain the existing behavior using a new time-set.target leaving
time-sync.target for cases where accuracy is required.
Closes#8861
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
Peter A. Bigot