Add an AC line monitor so power_profile can work

[jhibbits]

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.

[nwhitehorn]

Nice work! I do wonder (this is out of scope, however) whether it is worth standardizing some of these interfaces. Hopefully various ARM hardwares (Chromebooks, for example) will acquire the same features.

[rpaulo]

What happens if you remove the battery? Shouldn't the thread have some condition variable to let it terminate?

[jhibbits]

In D937#6, @rpaulo wrote:

What happens if you remove the battery? Shouldn't the thread have some condition variable to let it terminate?

There is one enhancement I want to make, to both this and the pmac_thermal thread, and that is to suspend the thread via a sysctl, instead of, in the case of pmac_thermal, just idling every second.

If you remove the battery, you would still depend on the AC line, so this wouldn't matter. I tested just now, that if you boot with no battery, it still detects that a battery is allowed, so the thread starts up.

[nwhitehorn]

What happens on systems with no battery? For example, desktops?

[jhibbits]

On systems where a battery will never be found, the thread doesn't start up. It's the same logic as adding the sysctls.

[jhibbits]

Closed by commit rS273009 (authored by @jhibbits).