manager: apply clock epoch on updates too

manager: apply clock epoch on updates too

If we're updating on a system with an invalid clock, and we're installing
a newer system version with a higher update, adjust the clock. This
way the invariant that the clock is always later than
max(compile time, timestamp file, other timestamp file) is maintained.

Also, adjust the wording of messages. When /usr/lib/clock-epoch was
introduced, "build time" stopped being acurate. Just say "epoch" instead.