Fixed rtnetlink attributes packaging into netlink message.

I've also added NetworkLinkDel() function which allows deleting
of the existing network links.

Docker-DCO-1.1-Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com>
(github: milosgajdos83)
Merge branch 'tenus-libcontainer' of github.com:milosgajdos83/libcontainer into tenus-libcontainer