Impact: change (fix) the way the NTP PLL seconds offset is initialized/tracked

Fix a bug and do a micro-optimization:

When PLL is enabled we do not reset time_reftime. If the PLL
was off for a long time (for example after bootup), this is
arguably the wrong thing to do.

We already had a hack for the common boot-time case in
ntp_update_offset(), in form of:

	if (unlikely(time_status & STA_FREQHOLD || time_reftime == 0))
 		secs = 0;

But the update delta should be reset later on too - not just when
the PLL is enabled for the first time after bootup.

So do it on !STA_PLL -> STA_PLL transitions.

This changes behavior, as previously if ntpd was disabled for
a long time and we restarted it, we'd run from that last update,
with a very large delta.
Signed-off-by: Ingo Molnar <mingo@elte.hu>