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 This is the only place where the config is gzipped. The kernel configs found in other places are uncompressed.

This is why reading the kernel config automatically was failing for me.