zmq_poll(): Rewrite to use ZMQ_FD/ZMQ_EVENTS pt2

Rewrite the select()-based zmq_poll() implementation to use
ZMQ_FD and ZMQ_EVENTS.

Also fix some corner cases: We should not pollute revents with
unrequested events, and we don't need to poll on ZMQ_FD at all
if a pollitem with no events set was passed in.