dgram: refactor SO_RCVBUF and SO_SNDBUF methods

This commit refactors the get/set send/receive buffer size
methods in the following ways:

- Use booleans instead of strings and numbers to differentiate
between the send and receive buffers.
- Reduce the amount of branching and complexity in the C++ code.

Refs: https://github.com/nodejs/node/pull/13623
PR-URL: https://github.com/nodejs/node/pull/15483


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>