Reorganize and clarify the DOCKER_BUILDTAGS docs

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add AUDIT_WRITE cap

Fixes #6345

Thanks @larsks for outstanding investigation

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
[DOCS] replace foo/bar with concrete names

namespaces are not well documented, and I had to jump around to other docs. replacing `foo/bar` hopefully makes what's going on here a bit more obvious.

Docker-DCO-1.1-Signed-off-by: Tim Ruffles <timruffles@gmail.com> (github: timruffles)
Typo changes to docker-run-1.md - changing '-' to '--' where approp.

Docker-DCO-1.1-Signed-off-by: William Henry <whenry@redhat.com> (github: ipbabble)
Merge pull request #7181 from timruffles/patch-3

[DOCS] replace foo/bar with concrete names
Merge pull request #7148 from tianon/buildtags-docs

Reorganize and clarify the DOCKER_BUILDTAGS docs
Merge pull request #7199 from ipbabble/mp-changes

Fixed some typos for '--' Vs '-' in docker-run-1.md
Properly handle 0 being passed as a signal

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Merge pull request #7179 from LK4D4/add_cap_audit_write_#6345

Add AUDIT_WRITE cap
Merge pull request #7212 from mheon/kill_zero

Properly handle 0 being passed as a signal number
update go import path and libcontainer

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
gofmt -s -w

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
revert AUTHORS changes

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #7225 from vieux/update_path

update go import path and libcontainer
Extract log utils into pkg/log
Output info to stdout
Only print Caller details on error case priorities
Use package level const instead of syslog
Remove log.New from pkg/log as it's only used in test code.

Restore integration/runtime_test.go's dependency on stdlib log and rename the import.