dgram: hide underscored Socket properties

dgram sockets have a fair number of exposed private properties.
This commit hides them all behind a single symbol property.

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>