time_settime() bugfix  (effects GPS-timekeeping)

I noticed that the seconds were jumping around on my GPS-enabled clock
today.  I believe this is because time_settime() pauses the
asynchronous counter that keeps track of time.  After restarting the
timer, the quartz crystal oscillator seems to take a few seconds to
stabilize.  During those seconds, the clock time would drift and be
again corrected by GPS, starting another cycle.

Assuming my explanation is correct, this bug has been fixed.

Unfortunately, the bug is nondeterministic, so although my GPS-enabled
clock is now working well, I will keep a close eye on it in the coming
days...