Disable rootless mode except RootlessCgMgr when executed as the root in userns

This PR decomposes `libcontainer/configs.Config.Rootless bool` into `RootlessEUID bool` and
`RootlessCgroups bool`, so as to make "runc-in-userns" to be more compatible with "rootful" runc.

`RootlessEUID` denotes that runc is being executed as a non-root user (euid != 0) in
the current user namespace. `RootlessEUID` is almost identical to the former `Rootless`
except cgroups stuff.

`RootlessCgroups` denotes that runc is unlikely to have the full access to cgroups.
`RootlessCgroups` is set to false if runc is executed as the root (euid == 0) in the initial namespace.
Otherwise `RootlessCgroups` is set to true.
(Hint: if `RootlessEUID` is true, `RootlessCgroups` becomes true as well)

When runc is executed as the root (euid == 0) in an user namespace (e.g. by Docker-in-LXD, Podman, Usernetes),
`RootlessEUID` is set to false but `RootlessCgroups` is set to true.
So, "runc-in-userns" behaves almost same as "rootful" runc except that cgroups errors are ignored.

This PR does not have any impact on CLI flags and `state.json`.

Note about CLI:
* Now `runc --rootless=(auto|true|false)` CLI flag is only used for setting `RootlessCgroups`.
* Now `runc spec --rootless` is only required when `RootlessEUID` is set to true.
  For runc-in-userns, `runc spec`  without `--rootless` should work, when sufficient numbers of
  UID/GID are mapped.

Note about `$XDG_RUNTIME_DIR` (e.g. `/run/user/1000`):
* `$XDG_RUNTIME_DIR` is ignored if runc is being executed as the root (euid == 0) in the initial namespace, for backward compatibility.
  (`/run/runc` is used)
* If runc is executed as the root (euid == 0) in an user namespace, `$XDG_RUNTIME_DIR` is honored if `$USER != "" && $USER != "root"`.
  This allows unprivileged users to allow execute runc as the root in userns, without mounting writable `/run/runc`.

Note about `state.json`:
* `rootless` is set to true when `RootlessEUID == true && RootlessCgroups == true`.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

libcontainer/specconv/spec_linux_test.go

@@ -417,7 +417,7 @@ func TestDupNamespaces(t *testing.T) {
 	}
 }
 
-func TestRootlessSpecconvValidate(t *testing.T) {
+func TestNonZeroEUIDCompatibleSpecconvValidate(t *testing.T) {
 	if _, err := os.Stat("/proc/self/ns/user"); os.IsNotExist(err) {
 		t.Skip("userns is unsupported")
 	}
@@ -430,7 +430,8 @@ func TestRootlessSpecconvValidate(t *testing.T) {
 		CgroupName:       "ContainerID",
 		UseSystemdCgroup: false,
 		Spec:             spec,
-		Rootless:         true,
+		RootlessEUID:     true,
+		RootlessCgroups:  true,
 	}
 
 	config, err := CreateLibcontainerConfig(opts)