Allow PM backends to report supported sleep types when registering through `power_pm_register`. Expose this information through `kern.power.supported_stype` sysctl, and set defaults for `power_standby/suspend/hibernate_stype` based on this. Implement this in ACPI PM backend.
This commit had to be reverted previously. The previous revision was D52044. The only change this revision makes it to use s2mem by default if a PM backend supports both s2idle and s2mem.