Rename ZMQ_MAX_SOCKETS_MAX to ZMQ_SOCKET_LIMIT

As per suggestion by Pieter Hintjens. Also update wording in
zmq_ctx_set manual a bit.
Merge pull request #892 from olafmandel/ZMQ_SOCKET_LIMIT

Rename ZMQ_MAX_SOCKETS_MAX to ZMQ_SOCKET_LIMIT