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)
Move UserLookup functionality into a separate pkg/user submodule that implements proper parsing of /etc/passwd and /etc/group, and use that to add support for "docker run -u user:group" and for getting supplementary groups (if ":group" is not specified)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Use type switch instead of reflection

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Consolidate a bunch of the supplementary-groups tests to cut down on the number of RUN lines in a single Dockerfile within TestBuild

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update lxc_template.go

If networking is disabled, but then pipework is used later to add nics, the network still doesn't function. Using flags=up for empty networking fixes this.
Docker-DCO-1.1-Signed-off-by: James DeFelice <james.defelice@ishisystems.com> (github: jdef)
remove mention of #docker-meeting

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
devmapper: Rename DeviceSet.RemoveDevice to DeleteDevice

"Remove" is a bit overloaded, as it is also what deactivating a
device mapper device is called. Using "delete" is more clear here.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: add and use removeDeviceAndWait helper

This adds a function that calls the lowlevel removeDevice
and then waits for it to finish.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
update issue filing & PR submission instructions

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
devmapper: Move refcounting to DeviceSet

We already have some kind of refcounting in DeviceSet, this fleshes
it out to allow it to completely subsume the refcounting in
devmapper.Driver.

This allows us to drop the double refcounting, and the locking inside
devmapper.Driver. This, in particular the locking simplification will
make it easier in the future to parallelize the device mapper.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Use removeDeviceAndWait in DeviceSet.removeDevice()

This makes sure the device is removed just like in deactivateDevice.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Handle EBUSY while removing

For some reason we seem to get transient EBUSY when removing
thinp devices, which prohibit removing containers. When
this happens we retry a few times which seems to fix the
issue for me.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Add json content type to /containers/json api endpoint

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Add "ONBUILD" highlighting to our vim syntax file

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3963 from unclejack/update-contributing

Update issue filing & PR submission instructions
Merge pull request #3974 from creack/3967-add_content_type-fix

Add json content type to /containers/json api endpoint
Added Dockerfile.tmPreferences for hotkey commenting

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)
Removed the YAML file since I wasn't using it

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)
Added suppport for the ONBUILD instruction

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)
Updated readme

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)