OHAI-491: Implement root_group for Windows 

Due to Windows-only bug [OHAI-490](https://tickets.opscode.com/browse/OHAI-490), the root_group plug-in was disabled for Windows. This change restores the functionality, and uses a purely local SID translation operation to avoid the issue we hit in the earlier implementation based on an inefficient WMI provider for the Win32_Group class. The LookupAccountSid method used in this implementation does a purely local lookup on one particular well-known SID, the one for the administrators group, and translates it to a name.

So the number of groups defined by the local system is not relevant, and certainly groups defined in AD will have no impact on the performance -- it's a simple local lookup of one SID.
I changed it to FFI::MemoryPointer.new(:pointer) -- thanks for catching.