dgram: refactor dgram to module.exports

Refactor dgram module to use the more efficient
module.exports = {} pattern.

PR-URL: https://github.com/nodejs/node/pull/11696


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>