General sleep state change clean up.

• Probe supported sleep states from acpi_attach() just once and do not

call AcpiGetSleepTypeData() again. It is redundant because
AcpiEnterSleepStatePrep() does it any way.

• Treat UNKNOWN sleep state as NONE, i.e., "do nothing", and remove obscure

NONE state (ACPI_S_STATES_MAX + 1) to avoid confusions.

• Do not set unsupported sleep states as default button/switch events.

If the default sleep state is not supported, just set it as UNKNOWN/NONE.

• Do not allow sleep state change if the system is not fully up and running.

This should prevent entering S5 state multiple times, which causes strange
behaviours later.

• Make sleep states case-insensitive when they are used with sysctl(8).

For example,

sysctl hw.acpi.lid_switch_state=s1
sysctl hw.acpi.sleep_button_state=none

are now legal and equivalent to the uppercase ones.