Use CGO for apparmor profile switch

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Add buildflags to allow crosscompilation for apparmor

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Update build tags such that we can properly compile on all platforms (especially for packagers), and updated hack/PACKAGERS.md to mention the DOCKER_BUILDTAGS variable that will need to be set for binaries that might be used on AppArmor (such as Debian and especially Ubuntu)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7 from tianon/fix-hack-tags

Update build tags such that we can properly compile on all platforms
Merge pull request #4506 from creack/fix_apparmor

Use CGO for apparmor profile switch