Add an AC line monitor so power_profile can work

Summary:
Add a polling loop (1Hz) to monitor the battery and AC status, to notify devd
like ACPI does for power monitoring.  This allows /etc/rc.d/power_profile to
work on PowerPC laptops

Test Plan:
Tested on a Titanium PowerBook, configuring economy_cpu_freq and
performance_cpu_freq, disabling powerd.

Reviewers: #powerpc, nwhitehorn

Reviewed By: nwhitehorn

Subscribers: rpaulo

Differential Revision: https://reviews.freebsd.org/D937