units: add time-set.target

units: add time-set.target

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