porting this code to i386mp branch):
- call microset() early on each cpu so that calls to microtime()
before the first clock interrupt don't return trash. this manifested
itself as garbage runtimes in "ps" for kernel threads.
- avoid races between hardclock updating "time" and microset on a
different cpu reading it by adding a "microset_time" global which is
initialized from "time" on the primary cpu.
- call microset every hz ticks, not every hz+1 (cosmetic)
4aaf078a4a