add systemd MAINTAINERS file

	new file:   contrib/init/systemd/MAINTAINERS

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org> (github: lsm5)
use systemd socket-activation by default

	modified:   contrib/init/systemd/docker.service
	renamed:    contrib/init/systemd/socket-activation/docker.socket -> contrib/init/systemd/docker.socket
	deleted:    contrib/init/systemd/socket-activation/docker.service

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org> (github: lsm5)
systemd unitfile requires docker.socket

	modified:   contrib/init/systemd/docker.service

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org> (github: lsm5)
Merge pull request #7021 from lsm5/systemd-maintainer

Add systemd MAINTAINERS file and update docker.service to always use socket activation