Problem: zmq_ctx_get (ZMQ_MAX_SOCKETS) returns gibberish

Well, not gibberish, but 2^31 on Linux, which is useless. The code
should probably use getrlimit on Linux and other calls depending on
the system. For now I've set the ceiling at 64K.

src/ctx.cpp

@@ -208,7 +208,7 @@ int zmq::ctx_t::get (int option_)
         rc = max_sockets;
     else
     if (option_ == ZMQ_SOCKET_LIMIT)
-        rc = clipped_maxsocket (std::numeric_limits<int>::max());
+        rc = clipped_maxsocket (65535);
     else
     if (option_ == ZMQ_IO_THREADS)
         rc = io_thread_count;