of the last sensor update time got broken, doesn't show up with gps since
it updates often (more often than we read), but naddy ran into it with dcf.
record time of last sensor datum seperately. ok naddy balmer

time, and make ntpd use that to send the next uery to an ntp peer and the
like. this has the advantage that changes to the clock do not interfere
with the intervals. for example, when we start on machines without an
RTC and the initial settime (-s) kicks in, intervals were strange.
idea from amandal@entrisphere.com, this implementation by me
tested ckuethe, phessler, mbalmer, ok mbalmer

them only every 30 seconds. now query them every 5,and take the median
value from 7 queries as sensor value. this takes outliers out of the
equation and makes the overall result much better, especially for
sensors with heavy jitter (like nmea for now)

From: amandal@entrisphere.com

-1 because of error, it may still participate in poll()
causing poll() to repeatedly wake up on error fd.
so make sure w edon't add -1 fds to pollevents to avoid unnecessary wakeups
From: amandal@entrisphere.com

From: amandal@entrisphere.com

too. from amandal@entrisphere.com

servers never update the system clock

takes the offset it  adjtime() is already correcting for into account
when taking the offset from a sensor, we need to correct it by the
offset between system time and ntpd view.

ok jmc@ henning@

david@; ok henning@

a few frequency adjustments. ok henning@

up causes overcompensation and confusing debug log entries; noticed
by dtucker@

henning@

ok henning@ ckuethe@

more than one token MUST be put in parentheses!

only do frequency compensation if the clock is synced, and a slightly
diffent way of computing the linear regression.
You'll need a recent kernel and libc to use this.
Testing by naddy@ and ckuethe@ and others, thanks!
ok henning@

ok henning@

overcompensating. From DragonFly, uses recent adjtime(2) changes,
so you'll need a recent kernel. ok henning@

ok henning@

ULONG_MAX does not work; the max upper bound is LONG_MAX,
since LONG_MAX == LLONG_MAX on 64bit archs. ok deraadt@ henning@

it will wait until the next poll.
ok henning@

from henning and myself

with ckuethe

weight timedelta sensors higher than ntp peers, for example
ok deraadt mbalmer

unfortunately I still don't have any hardware to test this ;(

are not a timedelta sensor any more on query on the fly anyway