Yet more scheduler work. Revamp the batch detection heuristic and fix a few

Yet more scheduler work.  Revamp the batch detection heuristic and fix a few
things here and there.  Test the algorithm against buildworlds, mozilla,
gnome-terminal, and pine all running at once.

* Fix a bug in how I handle p_slptime.
* Move the ++p_cptime out of usched_bsd4 and into kern_clock.c.
* Redo the batch heuristic, boost the cost of doing a fork, and
  slowly decrement p_origcpu to compensate for long-running
  processes.
* Use an instantanious load instead of a time-averaged load in the
  decay calculation.

TODO: the SMP support will be virtually identical across all schedulers
and should probably be abstracted out of usched_*.c.