Use flag for init

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Use a custom pipe instead of stdin for sync net namespace

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Minor cleanup

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Handle non-tty mode

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)