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)
Rearrange the existing info a little, and add example style guide

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5587 from SvenDowideit/docs-examples-style-guide

Rearrange the existing info a little, and add example style guide
Fix docker man page to reference selinux-enable flag

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)
Several reader issues fixed

- Fix boot2docker url
- move HomeBrew instructions to a separate section
- fix docker client 5-liner to work (its still ugly)
- fix and update program output

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5592 from SvenDowideit/docs-simplify-osx-install

Several OSX install reader issues fixed
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
nat: Allow "/" inside port value

This is a no-op for tcp/udp ports, but will be required for unix domain
sockets.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Support unix domain socket paths as exposed port for linking

If you expose a port of the "unix" protocol that like this:

docker run --expose /some/dir/socket_file/unix image

Then the port (/some/dir/socket_file in the above) will be mounted
into any container that links to the container. If the socket file
does not exist or is not a socket the linked container will fail
to start.

Here is an example of using this:
docker run -d --name test --expose /socket/unix fedora dbus-daemon --session --address unix:path=/socket
docker run -t -i --link test:test fedora dbus-monitor --address unix:path=/socket

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)