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.
I cleaned up string references, whitespace, and use of unstable
features. I also added a comment about reverting to connect, making
others aware that the method should be replaced by join after 1.3.