bug#61884: add an option to du that allows to control which file types are counted

[Paul Eggert]

What's the motivation here? Does this have something to do with 
reproducible builds?

One possibility is for --apparent-size to always count 0 for 
directories, since 'read' never returns a positive number on 
directories. That is, we reinterpret --apparent-size to mean "bytes that 
could be read" rather than "what st_size says".