libct: Signal: honor RootlessCgroups

`signalAllProcesses()` depends on the cgroup and is expected to fail
when runc is running in rootless without an access to the cgroup.

When `RootlessCgroups` is set to `true`, runc just ignores the error
from `signalAllProcesses` and may leak some processes running.
(See the comments in PR 4395)
In the future, runc should walk the process tree to avoid such a leak.

Note that `RootlessCgroups` is a misnomer; it is set to `false` despite
the name when cgroup v2 delegation is configured.
This is expected to be renamed in a separate commit.

Fix issue 4394

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

libcontainer/state_linux.go

@@ -44,6 +44,7 @@ func destroy(c *Container) error {
 	// and destroy is supposed to remove all the container resources, we need
 	// to kill those processes here.
 	if !c.config.Namespaces.IsPrivate(configs.NEWPID) {
+		// Likely to fail when c.config.RootlessCgroups is true
 		_ = signalAllProcesses(c.cgroupManager, unix.SIGKILL)
 	}
 	if err := c.cgroupManager.Destroy(); err != nil {