MP Implmentation 4/4: Final cleanup for this stage. Deal with a race

MP Implmentation 4/4: Final cleanup for this stage.  Deal with a race
that occurs due to not having to hold the MP lock through an lwkt_switch()
where another cpu may pull off a process from the userland scheduler and
schedule its thread before the original cpu has completely switched out it.
Oddly enough latencies were enough that this bug never caused a crash!

Cleanup the scheduling code and in particular the switch assembly code, save
and restore eflags (cli/sti state) when switching heavy weight processes
(this is already done for light weight threads), add some counters, and
optimize fork() to (statistically) stay on the current cpu for a short while
to take advantage of locality of cache reference, which greatly improves
fork/exec times.  Note that synchronous pipe operations between two procseses
already (statistically) stick to the same cpu (which is what we want).