check for errors in call, not readBody

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
vendor: add github.com/coreos/go-systemd/activation

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
server: add socket activation

This adds the ability to socket activate docker by passing in
`-H fd://*` along with examples systemd configuration files.

The fastest way to test this is to run:

```
/usr/lib/systemd/systemd-activate -l 127.0.0.1:2001 /usr/bin/docker -d -H 'fd://*'
docker -H tcp://127.0.0.1:2001 ps
```

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
fix(docs): fixup based on changes in master

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
docs: improve the socket activation cli docs

as suggested by SvenDowideit expand the docs to have more information on
socket activation.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
fix(docs): add Systemd in front of socket activation

Suggested by SvenDowideit.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Move listenfd to utility package

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Allow fd:// like unix:// and tcp://

Somthing like 20605eb310f0b57bd06eea80ec63c5022fc83bde

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
fix(cli.rst): add missing underscores

As suggested by @metalivedev

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
fix(contrib/init/systemd): remove mount rprivate

Docker does this now via 157d99a72786c454dfaad8b0800914cc80879aa8

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
chore(*): go fmt

I noticed that travis was failing, go fmt to make it happy.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
vendor: bump github.com/coreos/go-systemd/activation

tests now work in the Docker tree with

`go test github.com/coreos/go-systemd/activation`

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
chore(systemd): use activation.Listeners instead of Files

Use this Listeners() API that was exposed to save a few more lines of
boiler plate code.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
chore(coreos/go-systemd): copy to github.com/dotcloud/docker/systemd/pkg/activation

Via https://github.com/dotcloud/docker/pull/3105#issuecomment-32807547

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
fix(api): , _ removed to simplify code in range

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Add --ip-forward flag to daemon (enabled by default) which automatically sets "net.ipv4.ip_forward" to 1

See also https://groups.google.com/d/topic/docker-dev/DCjF5Prx7HA/discussion

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3801 from tianon/ip-forward

Add --ip-forward flag to daemon (enabled by default)
remove ip_forward warning

Now that docker sets /proc/sys/net/ipv4/ip_forward by default (unless
the user manually specifies "-ip-forward=false"), there's no need to
warn if its disabled.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3811 from jpoimboe/remove-ip_forward-warning

remove ip_forward warning
Merge pull request #3105 from philips/add-socket-activation

Add socket activation