check if the daemon is run as root on startup

This commit makes Docker throw an error if the daemon isn't started as
root.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5496 from unclejack/check_if_root

check if the daemon is run as root on startup