networking.md: Networking between containers works without ip_forward

Docker-DCO-1.1-Signed-off-by: Erik Inge Bolsø <knan@redpill-linpro.com> (github: knan-linpro)
 #1852 Remote API documentation incorrect regarding private registries

Issues addressed:

- Remote API doco refers to "registry" parameter which is not supported
- Neglects to mention "tag" parameter which is supported
- Offers no guidance on pushing to private registries

Changes made:

- As directed in review comments, `registry` removed from all api versions (1.0-1.14)
- Have added documentation for `tag` parameter and guidance on private registries for versions 1.10-1.14 (I have only validated this on .10)

Docker-DCO-1.1-Signed-off-by: Lachlan Coote <lachlan.t.coote@gmail.com> (github: lcoote)
Purge the bits of pkg/system that moved to libcontainer/system

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Fix example of relative WORKDIR

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #7363 from lcoote/master

 #1852 Remote API documentation incorrect regarding private registries
Merge pull request #7391 from LK4D4/fix_relative_workdir_example

Fix example of relative WORKDIR
Update libcontainer to 68ea1234a0b046803aacb2562df

Signed-off-by: Michael Crosby <michael@docker.com>
Ensure thread is locked in sysinit for docker

Signed-off-by: Michael Crosby <michael@docker.com>
Merge pull request #7407 from crosbymichael/update-libcontainer-aug1

Update libcontainer to version 68ea1234a0b046803aacb2562df0da1
Merge pull request #7388 from tianon/remove-pkg-system

Purge the bits of pkg/system that moved to libcontainer/system
Correct grammer in MAINTAINERS.md

Signed-off-by: Stephen Crosby <stevecrozz@gmail.com>
Add a "daemon" build tag and toggle it with the already-existing "DOCKER_CLIENTONLY" build variable

This works mostly by refactoring our "main" package to be careful about what it imports based on the daemon build tag. :)

Also, I've updated Travis to test "client-only" compilation after it tests the daemon version.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Adjust build tags to be client-only in "make cross" too

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7310 from stevecrozz/maintainers-grammar-update

Correct grammar in MAINTAINERS.md
Merge pull request #7377 from tianon/daemon-build-tag

Add a "daemon" build tag and toggle it with the already-existing "DOCKER_CLIENTONLY" build variable
Merge pull request #7349 from knan-linpro/master

networking.md: Networking between containers works without ip_forward