dgram: make send cb act as "error" event handler

Modifies the dgram send() method to not emit errors when a DNS
lookup fails if there is a callback. Given that the same UDP
socket can be used to send messages to different hosts, the socket
can be reused even if one of those send() fails.

This slightly changes the behavior of a stable API, so that it behaves
as users would expect to.

This is based on https://github.com/joyent/node/pull/7738, which
landed in 77266d7fadd8dfefb107ccb1e3fe97f9620f1288.

Fixes: https://github.com/joyent/node/issues/4846
Refs: https://github.com/joyent/node/pull/7738
PR-URL: https://github.com/nodejs/io.js/pull/1796


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>