libct/cg: simplify getting cgroup manager

1. Make Rootless and Systemd flags part of config.Cgroups.

2. Make all cgroup managers (not just fs2) return error (so it can do
   more initialization -- added by the following commits).

3. Replace complicated cgroup manager instantiation in factory_linux
   by a single (and simple) libcontainer/cgroups/manager.New() function.

4. getUnifiedPath is simplified to check that only a single path is
   supplied (rather than checking that other paths, if supplied,
   are the same).

[v2: can't -> cannot]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/checkpoint_test.go

@@ -62,7 +62,7 @@ func testCheckpoint(t *testing.T, userns bool) {
 	}
 
 	config := newTemplateConfig(t, &tParam{userns: userns})
-	factory, err := libcontainer.New(t.TempDir(), libcontainer.Cgroupfs)
+	factory, err := libcontainer.New(t.TempDir())
 	ok(t, err)
 
 	container, err := factory.Create("test", config)