ULE/yielding patch for testing.

Enclosed is a patch that does two things:

1)  Reduces UP context switch time by over 10% making it faster than 4BSD 
on UP.  On SMP it's hard to compare since ULE can do as many as 30x as 
many switches per second on my 8way system.

2)  Restores old sched_yield() behavior from 6.x.  This was changed in 
-current unintentionally I think.

I'd appreciate any extra testing.  The ULE context switch time 
improvements required some changes to the frequency that we recalculate 
priorities.  I'm mostly interested in hearing whether this causes any 
regression in normal workloads.

Those of you still using 4BSD can also verify that the yield changes don't 
cause any problems there.

Thanks,
Jeff