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>
