ntpd: decrease MIN_FREQHOLD by 2, increase "penalty" for largish offset x2

ntpd: decrease MIN_FREQHOLD by 2, increase "penalty" for largish offset x2

> 2018-07-25:
> ntpd: increase MIN_FREQHOLD by 3
> This means we'll start correcting frequency ~5 minutes after start,
> not ~3.5 ones.
> With previous settings I still often see largish ~0.7s initial offsets
> only about 1/2 corrected before frequency correction kicks in,
> resulting in ~200ppm "correction" which is then slowly undone.

Review of real-world results of the above shows that with small
initial offsets, freq correction can be allowed to kick in sooner,
whereas with large (~0.8s) offsets, we still start freq correction
a bit too soon.

Let's rebalance this a bit.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>