test,dgram: add tests for setBroadcast()

The only tests for `setBroadcast()` (from the `dgram` module) were in
`test/internet` which means they almost never get run. This adds a
minimal test that can check JS-land functionality in `test/parallel`.

I also expanded a comment and did some minor formatting on the existing
`test/internet` test. If there were an easy and reliable way to check
for the BROADCAST flag on an interface, it's possible that a version of
the test could be moved to `test/sequential` or `test/parallel` once it
was modified to only use internal networks.

PR-URL: https://github.com/nodejs/node/pull/6750


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>