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)
add _ping endpoint

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
add doc

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
return write error

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
nat: Fix --expose protocol parsing

A command like:
 docker run --expose 5353/tcp -P fedora sleep 10

Currently fails with:
Error: Cannot start container 5c558de5f0bd85ff14e13e3691aefbe531346297a27d4b3562732baa8785b34a: unknown protocol

This is because nat.SplitProtoPort() confuses the order of the port and
proto in 5353/tcp, assuming the protocol is first. However, in all other
places in docker the protocol is last, so the fix is just to swap these.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Fix docker man page to reference selinux-enable flag

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #5562 from rhatdan/master

Fix docker man page to reference selinux-enable flag
Merge pull request #5569 from vieux/add__ping_to_the_API

Add  ping to the api
Merge pull request #5496 from unclejack/check_if_root

check if the daemon is run as root on startup
Merge pull request #5591 from alexlarsson/fix-expose-parsing

nat: Fix --expose protocol parsing