Make Power Management work on MacBooks and make it configurable

Power Management: Something should happen on closing the lid on a MacBook Pro. At the very least, the screen should be switched off.

https://wiki.freebsd.org/SuspendResume says
> For the lid to operate suspend/resume set: `sysctl hw.acpi.lid_switch_state=S3`

We probably need to make Power Saving opt-in and configurable in a preferences application.

https://forums.freebsd.org/threads/how-to-make-the-laptop-sleep-when-lid-is-closed.6942/#post-40692 says
> Just set # sysctl hw.acpi.lid_switch_state=S3 **if you know** that ACPI state S3 (Suspend) works for you.

Could we get the information whether ACPI state S3 (Suspend) works for a particular machine from @linuxhw?