Move nsenter C code to separate file

Signed-off-by: Michael Crosby <michael@docker.com>
Add linux style formatting

Signed-off-by: Michael Crosby <michael@docker.com>
Merge pull request #144 from crosbymichael/move-c-code

Move nsenter C code to separate file
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)
Replaced log.Fatalf() with t.Fatalf()

No need to import log package.

Docker-DCO-1.1-Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com> (github: milosgajdos83)
Replacing t.Fatal() with t.Fatalf()

Docker-DCO-1.1-Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com> (github: milosgajdos83)
Merge pull request #139 from milosgajdos83/tenus-libcontainer

 RtAttr packaging fix. Added NetworkLinkDel() func and a new test.
Enter cgroups while entering namespaces as part of NsEnter.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Enter cgroups as part of nsenter while using systemd.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Refactoring cgroups enter.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Use strncmp.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)