Since several utilities check if the standard streams are interactive, I
moved this into the uucore::fs library as is_std*_interactive(). I also
added Windows support for these methods, which only return false (or at
least until someone finds a way to support this).
Everything in src/common has been moved to src/uucore. This is defined
as a Cargo library, instead of directly included. This gives us
flexibility to make the library an external crate in the future.
Fixes#717.
