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)
Upstream-commit: 87fb2c973d8f9a8a1868ab0c2da504095d04715b
Component: engine
