Simplify cgroup path handing in v2 via unified API

This unties the Gordian Knot of using GetPaths in cgroupv2 code.

The problem is, the current code uses GetPaths for three kinds of things:

1. Get all the paths to cgroup v1 controllers to save its state (see
   (*linuxContainer).currentState(), (*LinuxFactory).loadState()
   methods).

2. Get all the paths to cgroup v1 controllers to have the setns process
    enter the proper cgroups in `(*setnsProcess).start()`.

3. Get the path to a specific controller (for example,
   `m.GetPaths()["devices"]`).

Now, for cgroup v2 instead of a set of per-controller paths, we have only
one single unified path, and a dedicated function `GetUnifiedPath()` to get it.

This discrepancy between v1 and v2 cgroupManager API leads to the
following problems with the code:

 - multiple if/else code blocks that have to treat v1 and v2 separately;

 - backward-compatible GetPaths() methods in v2 controllers;

 -  - repeated writing of the PID into the same cgroup for v2;

Overall, it's hard to write the right code with all this, and the code
that is written is kinda hard to follow.

The solution is to slightly change the API to do the 3 things outlined
above in the same manner for v1 and v2:

1. Use `GetPaths()` for state saving and setns process cgroups entering.

2. Introduce and use Path(subsys string) to obtain a path to a
   subsystem. For v2, the argument is ignored and the unified path is
   returned.

This commit converts all the controllers to the new API, and modifies
all the users to use it.

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

libcontainer/container_linux_test.go

@@ -15,11 +15,10 @@ import (
 )
 
 type mockCgroupManager struct {
-	pids        []int
-	allPids     []int
-	stats       *cgroups.Stats
-	paths       map[string]string
-	unifiedPath string
+	pids    []int
+	allPids []int
+	stats   *cgroups.Stats
+	paths   map[string]string
 }
 
 type mockIntelRdtManager struct {
@@ -55,8 +54,8 @@ func (m *mockCgroupManager) GetPaths() map[string]string {
 	return m.paths
 }
 
-func (m *mockCgroupManager) GetUnifiedPath() (string, error) {
-	return m.unifiedPath, nil
+func (m *mockCgroupManager) Path(subsys string) string {
+	return m.paths[subsys]
 }
 
 func (m *mockCgroupManager) Freeze(state configs.FreezerState) error {