Error if Docker daemon starts with BTRFS graph driver and SELinux enabled

The Docker btrfs graph driver does not interact well with SELinux at present.
If btrfs mounts the same file in several locations, the same SELinux label will
be applied to all mountpoints. In the context of the graph driver, things such
as shared libraries become inaccessible to containers due to SELInux, causing
all dynamically linked applications to fail when run in a container.

Consequently, error when we detect the daemon is being run with SELinux enabled
and the btrfs driver. Documentation has been added for this behavior.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)

Conflicts:
	docker/docker.go