C2/C3 disabled by default

There were still a couple complaints about C3 failing, possibly on older 
laptops only.  However, since the failure case is a hang or poor runtime 
behavior, I thought it would be better to be safe and disable both C2 
and C3 for the release.  If we can nail down the problem laptops, it 
will be re-enabled.  For now, if you're using this, re-enable it in 
rc.conf with something like:

performance_cx_lowest="LOW"
economy_cx_lowest="LOW"

-- 
Nate