dgram: add getters/setters for private APIs

This commit makes all previously private APIs available via
getters, setters, and wrapper functions.

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>