Windows: Allow running as a service

Signed-off-by: John Howard <jhoward@microsoft.com>

Allows containerd.exe to run as a Windows service. eg

Register: `.\containerd.exe --register-service`

Start: `net start containerd`
...
Stop: `net stop containerd`

Unregister: `.\containerd.exe --unregister-service`

When running as a service, logs will go to the Windows application
event log.

services/server/server_test.go

@@ -17,7 +17,6 @@
 package server
 
 import (
-	"context"
 	"testing"
 
 	srvconfig "github.com/containerd/containerd/services/server/config"
@@ -25,9 +24,9 @@ import (
 	is "gotest.tools/assert/cmp"
 )
 
-func TestNewErrorsWithSamePathForRootAndState(t *testing.T) {
+func TestCreateTopLevelDirectoriesErrorsWithSamePathForRootAndState(t *testing.T) {
 	path := "/tmp/path/for/testing"
-	_, err := New(context.Background(), &srvconfig.Config{
+	err := CreateTopLevelDirectories(&srvconfig.Config{
 		Root:  path,
 		State: path,
 	})