zzz: Rewrite to use new power device

Previous script called acpiconf(8) (or apm(8) if ACPI wasn't supported,
although this was anyway redundant because APMIO just uses ACPI now).

Since a new generic power management interface was introduced, this isn't
sufficient, as this would only work for ACPI systems and for ACPI S3 suspend
(so no way to select suspend-to-idle). Rewrite in C to take advantage of the
new power interface.

We may want to add a switch to manually override the kern.power.suspend sysctl,
which is otherwise what the power device uses to decide which suspend type to
switch to (suspend-to-idle or firmware suspend), but this will require us to
amend the power interface.

Reviewed by: olce, imp, mhorne, ziaee
Tested by: mhorne
Approved by: olce, imp, mhorne, ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56918