Hi,
This is quite a big change from v8. Fixes a few bugs in child priority,
and adds a small lower bound on the amount of history that is kept. This
should improve "fork something" times hopefully, and stops new children
being able to fluctuate priority so wildly.

Eliminates "timeslice backboost" and only uses "priority backboost". This
decreases scheduling latency quite nicely - I can only measure 130ms for
a very low priority task, with a make -j3 and wildly moving an xterm around
in front of a mozilla window.

Makes a fairly fundamental change to how sleeping/running is accounted.
It now takes into account time on the runqueue. This hopefully will keep
priorities more stable under varying loads.

Includes an upper bound on the amount of priority a task can get in one
sleep. Hopefully this catches freak long sleeps like a SIGSTOP or unexpected
swaps. This change breaks the priority calculation a little bit. I'm 
thinking
about how to fix it.

Feedback welcome! Its against 0-test4, as usual.