By default, don't disable ACPI during reboot. This appears to hang some
systems. Introduce a new sysctl "hw.acpi.disable_on_reboot" that allows
users to re-enable the old behavior in case it's needed for some systems.
We never disable in the power-off path.

Original approach submitted by Alexander Logvinov <abuse@akavia.ru> with
reworking by Jung-uk Kim and myself.