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)
Add bidirectional iptables rule back to links

Fixes #4014
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4026 from crosbymichael/fix-linking-icc

Add bidirectional iptables rule back to links
docs: Installation Instruction Title & Description Fixes
Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)

1. All titles are listed by simple platform names apart from Windows' "Installing Docker On Windows". Changed this to "Windows" to match the rest.

2. Some articles' description (and title) information does not match the majority. Modified them to match the rest (i.e. Please note this project is currently under heavy development. It should not be used in production.)

3. Removed "Linux" from Gentoo & Ubuntu descriptions.
Merge pull request #3871 from jdef/patch-1

Update lxc_template.go
Merge pull request #3524 from tianon/supplementary-groups

Add supplementary groups lookup in sysinit
Merge pull request #4019 from ostezer/fix-installation-windows-doc-title

docs: Installation Instruction Title & Description Fixes