libpod: Factor out cgroups handling from (*Pod).refresh

This moves the cgroup code to pod_internal_linux.go and adds a no-op
stub for FreeBSD.

[NO NEW TESTS NEEDED]

Signed-off-by: Doug Rabson <dfr@rabson.org>

libpod/pod_internal_freebsd.go

@@ -0,0 +1,5 @@
+package libpod
+
+func (p *Pod) platformRefresh() error {
+	return nil
+}