Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()

The new options allows querying the maximum allowed number of sockets.
This is system dependent and cannot be encoded in the include file as a
preprocessor macro: for ZMQ_USE_SELECT, this depends on the FD_SETSIZE
macro at time of library compilation, not at time of include file use.
Merge pull request #889 from olafmandel/MAX_SOCKETS_MAX

Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()