Suspend to idle support

[bwidawsk]

Summary

This patch series implements suspend to idle. It was developed on github
here:
https://github.com/bwidawsk/freebsd/tree/suspend-to-idle

Suspend to idle is like s3 except instead of using ACPI to suspend and resume,
it simply idles the CPU. Please see the commit messages on github for a
description of the individual changes. In short, you mean specify S0IDLE for
certain sysctls to enable the support.

For example, sysctl hw.acpi.suspend_state=S0IDLE, will make acpiconf -s3 use
suspend to idle instead of the default (NONE or S3).

On top of this series will be support for s0ix (emulated s3), and then after
that, hopefully some sort of runtime power management. Emulated S3 support is
"done" and being debugged is here:
https://github.com/bwidawsk/freebsd/tree/s0ix

Test Plan

Set suspend to use suspend to idle, use power button to wake machine.

[jhb]

Is this one ready to be reviewed?

[bwidawsk]

I think so. I think there are definitely some robustifications needed, but generally it'd be good to know if I need any major overhauls. Also, at this point in the series, it's entirely opt-in, and any regressions should be fixable.