Add check-config to docker info

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

This PR rewrites check-config.sh in Go. The optional configs that are specific to storage drivers have been intentionally left out (@tianon).

This is again towards having a single `docker info` command that helps simple debugging.

If something's caught by CheckConfig it will be displayed as a WARNING at the end of `docker info`.

Ping @unclejack @tianon @crosbymichael @vieux
Should this file be `_linux` or `// +build linux` since it's linux-specific?