sys/power: Generic sleep types

Pull out the sleep types (stype) from ACPI, as was previously being
done in D48732, and pass this sleep type to power_pm_fn instead of
passing the existing sleep state. This is a little awkward because we
already kinda have generic sleep states (POWER_SLEEP_STATE_*), but
these are not precise enough to build upon.

This revision also adds generic equivalents to hw.acpi.suspend_state
etc sysctls, e.g. kern.power.suspend.

Reviewed by: markj, mckusick (mentor)
Approved by: markj, mckusick (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52036