cgroups: add GetFreezerState() helper to Manager

This is effectively a nicer implementation of the container.isPaused()
helper, but to be used within the cgroup code for handling some fun
issues we have to fix with the systemd cgroup driver.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/container_linux_test.go

@@ -61,10 +61,15 @@ func (m *mockCgroupManager) Path(subsys string) string {
 func (m *mockCgroupManager) Freeze(state configs.FreezerState) error {
 	return nil
 }
+
 func (m *mockCgroupManager) GetCgroups() (*configs.Cgroup, error) {
 	return nil, nil
 }
 
+func (m *mockCgroupManager) GetFreezerState() (configs.FreezerState, error) {
+	return configs.Thawed, nil
+}
+
 func (m *mockIntelRdtManager) Apply(pid int) error {
 	return nil
 }