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.

include/zmq.h

@@ -178,6 +178,7 @@ ZMQ_EXPORT void zmq_version (int *major, int *minor, int *patch);
 /*  Context options                                                           */
 #define ZMQ_IO_THREADS  1
 #define ZMQ_MAX_SOCKETS 2
+#define ZMQ_MAX_SOCKETS_MAX 3
 
 /*  Default for new contexts                                                  */
 #define ZMQ_IO_THREADS_DFLT  1