Add ability to signal container not just processes

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime/runtime.go

@@ -10,6 +10,7 @@ var (
 	ErrNotChildProcess      = errors.New("containerd: not a child process for container")
 	ErrInvalidContainerType = errors.New("containerd: invalid container type for runtime")
 	ErrCheckpointNotExists  = errors.New("containerd: checkpoint does not exist for container")
+	ErrCheckpointExists     = errors.New("containerd: checkpoint already exists")
 )
 
 // runtime handles containers, containers handle their own actions.