Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Since APM is a legacy technology, that won't be adapted to other
platforms, move the test for i386 from the Makefile to powerd.c. The
powerpc specific checks are already done this way...

The last big commit: let usr.sbin/ use WARNS=6 by default.

Don't use threads when there's no real reason to.

MFC after: 2 weeks

Use a pipe for reading devd events as another method of getting the AC line
status. Add a thread that waits for events on the named pipe instead of
polling the line status via apm or the ACPI sysctl. Additionally, use
robust error handling in case devd goes away or we temporarily can't set
a frequency (i.e., passive cooling has preempted our setting).

Later, this should be improved so that if adaptive control is not being used,
we will block while waiting for AC line events, saving a little CPU.

Submitted by: Frederik Lindberg <fli+freebsd-current/shapeshifter.se>

Add '-P' option which allows to specify pidfile.

Remove SRCS line since it isn't needed for single-source programs.
Pointed out by: ru

Import powerd(8), a utility for managing power consumption. Currently, it
just uses cpufreq(4) but in the future it should also do things like spin
down disks. This is based on the work by Colin Percival (called "estctrl").