podman pod create --pid flag

added support for --pid flag. User can specify ns:file, pod, private, or host.
container returns an error since you cannot point the ns of the pods infra container
to a container outside of the pod.

Signed-off-by: cdoern <cdoern@redhat.com>

libpod/runtime_pod_linux.go

@@ -116,6 +116,7 @@ func (r *Runtime) NewPod(ctx context.Context, options ...PodCreateOption) (_ *Po
 	if pod.config.UsePodCgroup {
 		logrus.Debugf("Got pod cgroup as %s", pod.state.CgroupPath)
 	}
+
 	if !pod.HasInfraContainer() && pod.SharesNamespaces() {
 		return nil, errors.Errorf("Pods must have an infra container to share namespaces")
 	}