dgram: make _createSocketHandle() internal only

_createSocketHandle() is used internally by the cluster module.
This commit makes it internal only API.

PR-URL: https://github.com/nodejs/node/pull/21923


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>