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>

restore.go

@@ -3,10 +3,11 @@
 package main
 
 import (
-	"fmt"
 	"os"
 
 	"github.com/opencontainers/runc/libcontainer"
+	"github.com/opencontainers/runc/libcontainer/system"
+	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
 )
 
@@ -96,12 +97,8 @@ using the runc checkpoint command.`,
 			return err
 		}
 		// XXX: Currently this is untested with rootless containers.
-		rootless, err := isRootless(context)
-		if err != nil {
-			return err
-		}
-		if rootless {
-			return fmt.Errorf("runc restore requires root")
+		if os.Geteuid() != 0 || system.RunningInUserNS() {
+			logrus.Warn("runc checkpoint is untested with rootless containers")
 		}
 
 		spec, err := setupSpec(context)