Add --foreground to docker start

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

Conflicts:
	runconfig/hostconfig.go
	server/server.go
Disable reuse-current in non-foreground mode

We really *never* want to reuse the docker daemon unit!

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)