IOStream doesn't detect remote hang up when _Select is used

[sris]

iostream.py provides a way to set a callback that should be invoked when a connection is closed. When using _Select this callback won't be invoked until a read or write is performed on the socket.

Select signals remote hang-ups by creating a read event with 0 byte length. This won't be noticed by iostream's _handle_event when a connection is idle since it only subscribes to io_loop.ERROR events.

This works fine with epoll since epoll with emit a HUP event that will get noticed.

[bdarnell]

In the select()-based IOLoop, always listen for reads so we can tell
when the other side closed the connection.

Closed by 1221865.