libeio bugfix part 3

Finally (hopefully) fix the issue that Felix reported. It's only appearing
on macintosh (test/mjsunit/test-eio-race3.js)

The trick/hack is to call eio_poll() again before reentering the event loop.

Additionally this commit implements a more complex method of calling
eio_poll(), occasionally dropping to an ev_idle watcher.

See also:
3f397728
http://lists.schmorp.de/pipermail/libev/2010q1/000855.html
http://groups.google.com/group/nodejs/browse_thread/thread/9f8db11c792a68bb/a89705f68971f53c