fix(CONTRIBUTING): Developer Certificate of Origin w/o Grant

In 7fb55f77250a9b52b0515a7e655ecc605ae452ca the DCO lost the grant so
change the title back to just DCO.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Adjust test to match its comment

Docker-DCO-1.1-Signed-off-by: Andrew Williams <williams.andrew@gmail.com> (github: TheDude05)
Make crosbymichael and creack netlink maintainers
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4186 from crosbymichael/netlink-maintainer

Make crosbymichael and creack netlink maintainers
Fix DOCKER_HOST=tcp:// panic

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Merge pull request #4030 from TheDude05/fix-incorrect-ip-test

Fix IPv4 Subnet Test
Merge pull request #4187 from creack/fix_hostflag_panic

Fix DOCKER_HOST=tcp:// panic
archive: Handle aufs plink hardlinks in ApplyLayer

Files in the .wh..wh.plnk directory are ignored, but other files
inside the tarfile can be hardlinks to these files. This is not
something that normally happens, as on aufs unmount such files are
supposed to be dropped via the "auplink" too, yet images on the index
(such as shipyard/shipyard, e.g. layer
f73c835af6d58b6fc827b400569f79a8f28e54f5bb732be063e1aacefbc374d0)
contains such files.

We handle these by extracting these files to a temporary directory
and resolve such hardlinks via the temporary files.

This fixes https://github.com/dotcloud/docker/issues/3884

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4178 from alexlarsson/fix-aufs-plnk

archive: Handle aufs plink hardlinks in ApplyLayer
Merge pull request #3817 from philips/CONTRIBUTING-is-just-a-dco

fix(CONTRIBUTING): Developer Certificate of Origin w/o Grant