power management, ahci link power management and DragonFly

[Matthew Dillon]

    Also make sure sysctl machdep.cpu_idle_hlt=2 is set.  There was a short
    period where I changed the default to 1 (which does not use C-states).
    2 should use C-states.  The current default is again 2.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>