ANNOUNCE: Thermal control for SMU-based G5 systems

[Nathan Whitehorn]

Fan control is now available on RELENG_8 and -CURRENT for Apple G5 
machines with an SMU (all except PowerMac 7,2 and 7,3), so such machines 
should no longer sound like they are full of angry bees. Liquid-cooling 
pumps are not currently supported, since I do not have liquid-cooled 
hardware handy.

Thanks to Peter Grehan for implementation advice!
-Nathan