"Fast" path in `uv_udp_send()` attempts to write the data straight away
without queueing the fd with epoll/kqueue/... However, in some cases
this is not possible when `sendmsg()` returns `EAGAIN`. In such event
libuv has to queue the fd anyway.
PR-URL: https://github.com/libuv/libuv/pull/1308
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
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