Update /etc/hosts when linked container is restarted

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
pkg/graphdb: tests for Parents and Children

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
pkg/networkfs/etchosts: tests for Update method

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Update networking doc with clarified text regarding links usage

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #7677 from erikh/update_hosts_linked_containers

Update /etc/hosts when linked container is restarted