If we're using a PIPT L2 cache, only merge 2 segments if both the virtual
and the physical addreses are contiguous.

Submitted by:	Thomas Skibo <ThomasSkibo@sbcglobal.net>
- Make callout(9) tickless, relying on eventtimers(4) as backend for
precise time event generation. This greatly improves granularity of
callouts which are not anymore constrained to wait next tick to be
scheduled.
- Extend the callout KPI introducing a set of callout_reset_sbt* functions,
which take a sbintime_t as timeout argument. The new KPI also offers a
way for consumers to specify precision tolerance they allow, so that
callout can coalesce events and reduce number of interrupts as well as
potentially avoid scheduling a SWI thread.
- Introduce support for dispatching callouts directly from hardware
interrupt context, specifying an additional flag. This feature should be
used carefully, as long as interrupt context has some limitations
(e.g. no sleeping locks can be held).
- Enhance mechanisms to gather informations about callwheel, introducing
a new sysctl to obtain stats.

This change breaks the KBI. struct callout fields has been changed, in
particular 'int ticks' (4 bytes) has been replaced with 'sbintime_t'
(8 bytes) and another 'sbintime_t' field was added for precision.

Together with:	mav
Reviewed by:	attilio, bde, luigi, phk
Sponsored by:	Google Summer of Code 2012, iXsystems inc.
Tested by:	flo (amd64, sparc64), marius (sparc64), ian (arm),
		markj (amd64), mav, Fabian Keil
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