MFcalloutng (r244355):
Make loadavg calculation callout direct. There are several reasons for it:
 - it is very simple and doesn't worth context switch to SWI;
 - since SWI is no longer used here, we can remove twelve years old hack,
excluding this SWI from from the loadavg statistics;
 - it fixes problem when eventtimer (HPET) shares interrupt with some other
device, and that interrupt thread counted as permanent loadavg of 1; now
loadavg accounted before that interrupt thread is scheduled.

Sponsored by:	Google Summer of Code 2012, iXsystems inc.
Tested by:	flo, marius, ian, Fabian Keil, markj
Make dummynet use new direct callout(9) execution mechanism.  Since the only
thing done by the dummynet handler is taskqueue_enqueue() call, it doesn't
need extra switch to the clock SWI context.

On idle system this change in half reduces number of active CPU cycles and
wakes up only one CPU from sleep instead of two.

I was going to make this change much earlier as part of calloutng project,
but waited for better solution with skipping idle ticks to be implemented.
Unfortunately with 10.0 release coming it is better get at least this.

Conflicts:
	sys/netpfil/ipfw/ip_dummynet.c
Make load average sampling asynchronous to hardclock ticks. This improves
measurement of load caused by time-related events still using hardclock.
For example, without this change dummynet, scheduling events each hardclock
tick, was always miscounted as load of 1.

There is still aliasing with events delayed by the new precision mechanism,
but it probably can't be avoided without moving this sampling from using
callout to some lower-level code or handling it in some other special way.

Reviewed by:	davide
Approved by:	re (marius)