PR# kern/42139: ACPI WMI: new driver

Import acpiwmi(4) from Jukka Ruohonen. From the PR:

Attached is a driver that implements ACPI WMI API:

http://www.microsoft.com/whdc/system/pnppwr/wmi/wmi-acpi.mspx

The WMI is used to provide a generic interface for OEMs to use certain
platform/laptop-specific additions to the standard ACPI in a somewhat
portable way. These can be hotkeys for additional buttons, different event
handlers (wireless kill switch, lid switch, etc.), and so on. At least HP
and Acer use it by default nowadays.

The benefit of this interface would be portability. For an example, instead
of hpqlb(4) that works only with certain HP models, we could have a generic
HP WMI-driver that should work in theory across all HP laptops. On many new
laptops WMI may also be the only way to access laptop/manufacturer-specific
features.

(jmcneill)