Core continued

* Fix build
* Fix basic errors at daemon launch

Good news:

```
$ docker -d&
$ docker net create skynet
INFO[0001] POST /v1.16/cmd
INFO[0001] +job net_create(skynet)
INFO[0001] -job net_create(skynet) = OK (0)
$ ip a | grep skynet: -A5
9: skynet: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
    link/ether 8a:b6:b3:4d:8c:20 brd ff:ff:ff:ff:ff:ff
    inet 10.0.42.1/16 scope global skynet
       valid_lft forever preferred_lft forever
    inet6 fe80::88b6:b3ff:fe4d:8c20/64 scope link
       valid_lft forever preferred_lft forever
```

Issues:
* `network.Controller` returns a `nil` `Network` instance, so we cannot do any operations on a created network
* When the `"default"` bridge interface already exists, daemon launch fails on [`BridgeDriver.CreateBridge`](https://github.com/shykes/docker/blob/e30974b2cf52c60ee383ff677f3666008381f577/extensions/simplebridge/driver.go#L212) with `netlink.LinkAdd` returning error "file exists".

I'll try to submit integration-cli tests next.