Support NDP proxying on bridge networks

[mostolog]

Proxying is enabled via the network label com.docker.network.bridge.ndp_proxy_interface=

Closes #1159
Overrides #1316

[aboch]

I see in most of bridge code we first check the flag value, and we set it to 1 only if needed.
I know we would do only one write if we set it regardless, but I think it is better to be consistent and check first.

[mostolog]

Do you mean to change https://github.com/mostolog/libnetwork/blob/907282e5195e7bfaf507da0375bcaaf95a618603/drivers/bridge/setup_ipv6.go#L139 ?:

    if ndpProxyConfPerm[0] != '1' {
        if err := ioutil.WriteFile(ndpProxyConfPath, []byte{'1', '\n'}, ndpProxyConfPerm); err != nil {
            logrus.Warnf("Unable to enable NDP proxying for interface %s: %v", config.NDPProxyInterface,err)
        }
    }

[aboch]

Yeah, to follow same logic we do for the default, for example

https://github.com/mostolog/libnetwork/blob/907282e5195e7bfaf507da0375bcaaf95a618603/drivers/bridge/setup_ipv6.go#L125

First read the value, then set it if not already set.

[mostolog]

It may be solved now

[aboch]

Thanks @mostolog for carrying this forward.
Thanks for carrying the original commit.
Regarding the extra 3 commits of yours, please squash them into one, and sign it with your id.

[aboch]

You now also need a rebase

[aboch]

Please fetch latest from upstream and rebase your changes. Otherwise we can't merge this.

[mostolog]

Already did git pull --rebase origin master and says it's up to date.

[aboch]

Do this

git fetch <remote>
git rebase <remote>/master

(As remote use the one which points to the github.com/docker/libnetwork repo, check with git remote -v)

[mostolog]

Current branch patch-1 is up to date.

[aboch]

This won't build

[aboch]

This has been in non-mergeable state for too long.

I am taking care of the merge conflicts and the modification of setupNDPProxying() to only enable the feature on the user specified host interface in #1714