Pass the get_real_filename operation through the VFS

This is done to help file systems that can tell us about the real upper/lower
case combination given a case-insensitive file name. The sample I will soon
push is the gpfs module (recent gpfs has a get_real_filename function), others
might have a similar function to help alleviate the 1million files in a single
directory problem.

Jeremy, please comment!

Thanks,

Volker