This will probably be superseded by the D52036, D52043, D52044 stack.
Borrow the idea to abstract over ACPI Sx sleep types with enum sleep_type from Ben's s2idle patch, D17675, but use this everywhere it makes sense to.
This is in preparation for an s2idle patch, which will add a new non-ACPI sleep state (STYPE_SUSPEND_TO_IDLE).