powerd adaptive mode latching

[Maxim Sobolev]

I wonder if somebody did measurement of power consumption with powerd 
and without it on typical tasks. There is very interesting idea in the 
last issue of ACM Queue that it might be much more beneficial to run CPU 
at the full speed and then switch it to low-power mode as soon as 
possible in the idle loop than to run longer at reduced speed for a 
longer period of time.

-Maxim