low HZ value causes "Time Warp Bug" (re: this Puny Pentium2 suddenly became 45% slower!)

It seems this bug happens when the HZ value goes below 16
(either by compiling 'options HZ=' in kernel or setting
sysctl 'kern.hz=' in /boot/loader.conf).  The computed
'ticks' value becomes too large for 2-byte int producing
crazy overflowed numbers elsewhere.

The crazyness extends to human clocks such as what KDE
uses.  I mean it was visibly speeding up and showing time
to go home before it was 'really' lunchtime!  Time warp!!

Something needs to check for overflow, but I bet whatever
this field is cannot easily be widend past 2-byte int?

I guess I will file a PR on this.  


  --  thx, Paul Seniura.