It was reported that some node.js tests fail on AIX because the exit
event sometimes comes before the final stdio output of a child process.
Work around that by deferring the signal watcher that is used for
process management until after the dispatch of regular i/o watchers.
Fixes: https://github.com/libuv/libuv/issues/610
PR-URL: https://github.com/libuv/libuv/pull/611
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
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