Add "apparmor_parser" warning to check-config.sh

If AppArmor is enabled on the current system, but "apparmor_parser" isn't installed, it causes all kinds of issues.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
pkg/units: Remove unused named returns

Remove named returns since not used in function body.  Might prevent
potential subtle bugs.

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)
Initial revisions to b2d doc, new screenshots.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

Final revisions to B2D doc.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #7169 from fredlf/update_osx-install

Initial revisions to b2d doc, new screenshots.
Merge pull request #7165 from fcarriedo/remove-named-returns

pkg/units: Remove unused named returns
Merge pull request #7157 from tianon/check-config-apparmor_parser

Add "apparmor_parser" warning to check-config.sh
pkg/units: Better to not use `error` as var name

Better to not use `error` as var name (might eclipse the error type) for
clarity and to prevent subtle bugs.

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)