Problem: older 4.1 libzmq's return nonsense for ZMQ_SOCKET_LIMIT

Solution: cut socket limit to 64K if it's larger. Otherwise using
it in zmq_ctx_set (ZMQ_MAX_SOCKETS) will exhaust process memory and
cause stupid things to happen.
Merge pull request #521 from hintjens/master

Problem: older 4.1 libzmq's return nonsense for ZMQ_SOCKET_LIMIT