Added option to keep containers running after checkpointing

CRIU supports to leave processes running after checkpointing:

  -R|--leave-running    leave tasks in running state after checkpoint

runc also support to leave containers running after checkpointing:

   --leave-running      leave the process running after checkpointing

With this commit the support to leave a container running after
checkpointing is brought to Podman:

   --leave-running, -R  leave the container running after writing checkpoint to disk

Now it is possible to checkpoint a container at some point in time
without stopping the container. This can be used to rollback the
container to an early state:

$ podman run --tmpfs /tmp --name podman-criu-test -d docker://docker.io/yovfiatbeb/podman-criu-test
$ curl 10.88.64.253:8080/examples/servlets/servlet/HelloWorldExample
3
$ podman container checkpoint -R -l
$ curl 10.88.64.253:8080/examples/servlets/servlet/HelloWorldExample
4
$ curl 10.88.64.253:8080/examples/servlets/servlet/HelloWorldExample
5
$ podman stop -l
$ podman container restore -l
$ curl 10.88.64.253:8080/examples/servlets/servlet/HelloWorldExample
4

So after checkpointing the container kept running and was stopped after
some time. Restoring this container will restore the state right at the
checkpoint.

Signed-off-by: Adrian Reber <areber@redhat.com>

libpod/container_api.go

@@ -833,7 +833,8 @@ func (c *Container) Refresh(ctx context.Context) error {
 // ContainerCheckpointOptions is a struct used to pass the parameters
 // for checkpointing to corresponding functions
 type ContainerCheckpointOptions struct {
-	Keep bool
+	Keep        bool
+	KeepRunning bool
 }
 
 // Checkpoint checkpoints a container