dgram: default send address to 127.0.0.1 or ::1

In net we default to 'localhost' as the default address for connect.
Not doing the same on dgram is confusing, because sending to 0.0.0.0
works on Linux/OS X but not on Windows. Defaulting that to 127.0.0.1 /
::1 addresses that.

Related: https://github.com/nodejs/node/pull/5407
Related: https://github.com/nodejs/node/issues/5398
Fixes: https://github.com/nodejs/node/issues/5487
PR-URL: https://github.com/nodejs/node/pull/5493


Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>