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
Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>