Correct RCVHWM value for zmq 3.x.
Correct SNDMORE type for zmq 3.x.

The type for zmq 2.x is actually also wrong, but uint64_t vs. int64_t
doesn't matter since it's a boolean value.
Poll supports -1 timeout.
Merge pull request #63 from somia/master

More 3.2 fixes.