Cleanup: Extract log utils into pkg/log

(RE: conversation in #docker-dev)

This is the beginning of a few pull requests to break up utils into more discrete packages. My team needs this so we can import packages from Docker without inadvertently inheriting all the imports from utils.

In our case, we need to break up some of utils so we don't accidentally pull in github.com/docker/docker/pkg/term, which does not compile on Windows.

I would prefer if you put that in your global gitignore.