ACPI: extend "acpi_osi=" boot option

The boot option "acpi_osi=" has always disabled Linux _OSI support,
thus disabling all OS Interface strings which are advertised
by Linux to the BIOS.

Now...
acpi_osi="string" adds the interface string, and
acpi_osi="!string" invalidates the pre-defined interface string

eg. acpi_osi="!Windows 2006"
would disable Linux's claim of Vista compatibility.

Signed-off-by: Len Brown <len.brown@intel.com>