Add GetInvalidOSIndependentFileNameChars method

On Unix there are more characters valid in file names, but we
want the result to be identical on both platforms, so we want
to use the larger invalid Windows list for both platforms.
This changes moves the method from Bloom to Palaso and renames
it to Path.GetInvalidOSIndependentFileNameChars().

Change-Id: Ia66459a8f3ee7e18aa9f353ad40a834d90f40943