POSIX defines `poll()` as a portable way to wait on file descriptors.
Use it to provide an implementation of our poll abstraction for use
on platforms that do not provide a more specialized polling method.
PR-URL: https://github.com/libuv/libuv/pull/1312
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
