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)

utils/utils.go

@@ -767,6 +767,8 @@ func ParseHost(defaultHost string, defaultPort int, defaultUnix, addr string) (s
 	case strings.HasPrefix(addr, "tcp://"):
 		proto = "tcp"
 		addr = strings.TrimPrefix(addr, "tcp://")
+	case strings.HasPrefix(addr, "fd://"):
+		return addr, nil
 	case addr == "":
 		proto = "unix"
 		addr = defaultUnix