Adding IPv6 network support

Docker-DCO-1.1-Signed-off-by: Malte Janduda <mail@janduda.net> (github: MalteJ)
Address6 -> IPv6Address and Gateway6 -> IPv6Gateway

Docker-DCO-1.1-Signed-off-by: Malte Janduda <mail@janduda.net> (github: MalteJ)
Merge pull request #203 from MalteJ/master

Adding IPv6 network support
Fix leaking file descriptor in NetNs strategy

Docker-DCO-1.1-Signed-off-by: Hugo Duncan <hugo@hugoduncan.org> (github: hugoduncan)
Merge pull request #205 from hugoduncan/fix-leaking-fd-in-netns

Fix leaking file descriptor in NetNs strategy