Patch to make powerd(8) devd(8)-aware

[Nate Lawson]

I appreciate you working on a patch for this.  However, I'd prefer the 
work on the devd side go into making devctl a clonable device or follow 
the bpf route and hook up /dev/devctl0,1,2,...  If you start using the 
/var/run/devd.pipe, it's not available for other consumers.

I agree that the sysctl errors should be soft at runtime although you 
should make sure that powerd(8) does not spin in a fast loop if sysctl 
returns an error (i.e. the sleep at the bottom should be called even 
when there's an error).

Other comments:
style(9) long sscanf line
extra newlines inserted into existing code
change read() in devd_read() to return on error instead of running 
sscanf -- this will allow the indent to be reduced.

-- 
Nate