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.
Remove destroy() methods.

Go does not automatically set destroy() methods as finalizers and no
code in gozmq calls runtime.SetFinalizer, nor does any code in gozmq
ever call one of the destroy() methods.