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.go

@@ -50,23 +50,28 @@ import (
 	"google.golang.org/grpc"
 )
 
-// New creates and initializes a new containerd server
-func New(ctx context.Context, config *srvconfig.Config) (*Server, error) {
+// CreateTopLevelDirectories creates the top-level root and state directories.
+func CreateTopLevelDirectories(config *srvconfig.Config) error {
 	switch {
 	case config.Root == "":
-		return nil, errors.New("root must be specified")
+		return errors.New("root must be specified")
 	case config.State == "":
-		return nil, errors.New("state must be specified")
+		return errors.New("state must be specified")
 	case config.Root == config.State:
-		return nil, errors.New("root and state must be different paths")
+		return errors.New("root and state must be different paths")
 	}
 
 	if err := os.MkdirAll(config.Root, 0711); err != nil {
-		return nil, err
+		return err
 	}
 	if err := os.MkdirAll(config.State, 0711); err != nil {
-		return nil, err
+		return err
 	}
+	return nil
+}
+
+// New creates and initializes a new containerd server
+func New(ctx context.Context, config *srvconfig.Config) (*Server, error) {
 	if err := apply(ctx, config); err != nil {
 		return nil, err
 	}