Fix dynbinary so that dockerinit can still be properly static even if it has to link against libapparmor for Ubuntu

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add new "DOCKER_CLIENTONLY" build variable to allow skipping of the dockerinit compilation, especially for Homebrew / Mac OS X client-only compilation

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4516 from tianon/fix-static-dockerinit

Fix dynbinary so that dockerinit can still be properly static even if it has to link against libapparmor for Ubuntu