Remove all darwin specific files and use more generic _unsupported with build tags.
Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Update/fix build tags, Dockerfile, and release.sh for proper building and releasing of linux/386 and linux/arm cross-compiled client binaries

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4 from tianon/cross-compile-and-release-linux-i386-and-arm-clients

Update/fix build tags, Dockerfile, and release.sh for proper building and releasing of linux/386 and linux/arm cross-compiled client binaries
Merge pull request #3857 from creack/remove_darwin_files

Remove all darwin specific files and use more generic _unsupported with build tags.