proc->thread stage3: make time accounting threads based and rework it for

proc->thread stage3: make time accounting threads based and rework it for
performance.

Cleanup user/sys/interrupt time accounting.  Get rid of the microputime and
equivalent support code in mi_switch() (it was really a bad idea to put that
in the critical path IMHO).  Instead account for time statistically
from the statclock, which produce time accounting that is just as accurate
in the long haul.  Remove the u/s/iticks fields from the proc structure and
put a slightly different version in the thread structure, so time can be
accounted for both threads and processes.