If securelevel>1, allow the clock to be adjusted negatively only up to
1 second prior to the highest the clock has run so far. This allows
time adjusters like xntpd to do their work, but the worst a miscreant
can do is "freeze" the clock, not go back in time.

We still need to decide on an algorithm to clamp positive adjustments.
As it stands, it is possible to achieve arbitrary negative adjustments
by "wrapping" time around.

PR: 10361