[patch] enhance powerd(8) to handle max temperature

[ports at fsck.ch]

> I'll take a look at your patch.  Umemoto-san is right in that you really
> want the kernel to control cooling.  What happens if powerd dies/hangs
> and your system burns up?  Passive cooling is often a last resort to
> keep the system from overheating.

Something like that happened to me once. I was playing around with the
thinkpad fancontrol utility and I turned the fan off. I started a
buildworld, forgot to turn the fan back on, and went to bed. The next
morning, the laptop was shut down, and I had a syslogged message along the
lines "CPU temp is 100 dec c, emergency shutdown". I don't know the exact
wording, but the 100 dec C was in there. No noticable impact to the
system.

Whew.