Removed heap allocation for in zmq_poll for small item sets

Until now, zmq_poll always allocates the poll items on the heap.
Now, small item sets, up to ZMQ_POLLITEMS_DFLT, are stack allocated
and only larger sets are allocated on the heap.
Merge pull request #507 from bjoto/master

 LIBZMQ-498 - Remove heap allocations in zmq_poll for small poll item sets