shim v2 hangs on reboot/shutdown

**Description**

Containerd is shipping with `KillMode=process` in systemd unit, so that shims won't be killed if containerd stops.
Systemd broadcast SIGTERM during the final stage of shutdown, this is the only chance for shim to terminate gracefully.
It seems that shim v2 does not handle SIGTERM/SIGINT at all, and hangs machine reboot/shutdown for 90s, got killed at last.

Shim v1 handles SIGINT/SIGTERM
https://github.com/containerd/containerd/blob/c7e4747cfb5cf15eef68af71b0a5526f2343f635/cmd/containerd-shim/main_unix.go#L248-L261

Shim v2 registers and ignores SIGINT/SIGTERM 
https://github.com/containerd/containerd/blob/c7e4747cfb5cf15eef68af71b0a5526f2343f635/runtime/v2/shim/shim_unix.go#L81-L87

See also:
* https://github.com/moby/moby/issues/41831
* https://github.com/k3s-io/k3s/issues/2400
* https://github.com/containerd/containerd/issues/386

**Steps to reproduce the issue:**
1. install docker-ce 20.10.x, enable live-restore
2. `docker run -d k8s.gcr.io/pause`
3. `sudo reboot`

**Describe the results you received:**
The shutdown/reboot process stuck for 90s, due to containerd-shim.
```
[  OK  ] Reached target Shutdown.
[  OK  ] Reached target Final Step.
[  OK  ] Finished Reboot.
[  OK  ] Reached target Reboot.
[  214.337805] systemd-shutdown[1]: Waiting for process: containerd-shim
```

**Describe the results you expected:**
containerd-shim should not interfere with shutdown/reboot.

**What version of containerd are you using:**
```
$ containerd --version
containerd containerd.io 1.4.4 05f951a3781f4f2c1911b05e61c160e9c30eaa8e
```


	case unix.SIGTERM, unix.SIGINT:
	go termOnce.Do(func() {
	ctx := context.TODO()
	if err := server.Shutdown(ctx); err != nil {
	logger.WithError(err).Error("failed to shutdown server")
	}
	// Ensure our child is dead if any
	sv.Kill(ctx, &shimapi.KillRequest{
	Signal: uint32(syscall.SIGKILL),
	All: true,
	})
	sv.Delete(context.Background(), &ptypes.Empty{})
	close(done)
	})

	switch s {
	case unix.SIGCHLD:
	if err := reaper.Reap(); err != nil {
	logger.WithError(err).Error("reap exit status")
	}
	case unix.SIGPIPE:
	}

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shim v2 hangs on reboot/shutdown #5502

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shim v2 hangs on reboot/shutdown #5502

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