unix: make uv_poll_stop() remove fd from pollset

Avoid repeated wake-ups in `uv__io_poll()` when the previously watched
file descriptor has been duplicated (with `dup(2)`) and then closed.

Because epoll reports events for _file descriptions_ rather than _file
descriptors_, events on the duplicated file descriptor cause the event
loop to wake up.

Libuv then tries to unregister the event listener for the original file
descriptor but that fails with EBADF because the file descriptor is
closed.

Since libuv uses epoll in level-triggered mode, it effectively results
in a busy loop because the event is re-reported as soon as libuv calls
epoll_wait() again.

I tried hard to write a test case for this but there is no directly
observable behavior, only increased CPU usuage.

Fixes: https://github.com/libuv/libuv/issues/1148
PR-URL: https://github.com/libuv/libuv/pull/1468
Reviewed-By: Colin Ihrig <cjihrig@gmail.com
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/poll.c

@@ -101,6 +101,7 @@ static void uv__poll_stop(uv_poll_t* handle) {
               &handle->io_watcher,
               POLLIN | POLLOUT | UV__POLLRDHUP | UV__POLLPRI);
   uv__handle_stop(handle);
+  uv__platform_invalidate_fd(handle->loop, handle->io_watcher.fd);
 }