make listen buffer optional

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
move acceptconnections as builtin

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5571 from vieux/fix_race_docker_daemon

Fix race docker daemon