ACPICA: Fixes for Unload and DDBHandles

Implemented support for the use of DDBHandles as an Indexed
Reference, as per the ACPI spec.

http://www.acpica.org/bugzilla/show_bug.cgi?id=486.

Implemented support for UserTerm (Method invocation) for the Unload operator
as per the ACPI spec.

http://www.acpica.org/bugzilla/show_bug.cgi?id=580

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>