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
@tiborvass Could you change this to make sure we print the list of locations we've tried to read the config from and only if we haven't found the config, please?

This is very noisy and might be confusing, making some believe something is missing.