Problem: no permission to relicense zmq_proxy_steerable

Solution: remove implementation. Laurent Alebarde <l.alebarde@free.fr>,
the author, did not respond to requests to allow relicensing to MPL2,
so we have to remove his copyrighted work.
Make the API into an empty stub that returns -EOPNOTSUPP.

doc/zmq_proxy_steerable.txt

@@ -3,7 +3,7 @@ zmq_proxy_steerable(3)
 
 NAME
 ----
-zmq_proxy_steerable - built-in 0MQ proxy with control flow
+zmq_proxy_steerable - DEPRECATED
 
 
 SYNOPSIS
@@ -14,101 +14,9 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-The _zmq_proxy_steerable()_ function starts the built-in 0MQ proxy in the 
-current application thread, as _zmq_proxy()_ do. Please, refer to this function
-for the general description and usage. We describe here only the additional 
-control flow provided by the socket passed as the fourth argument "control".
-
-If the control socket is not NULL, the proxy supports control flow. If 
-'PAUSE' is received on this socket, the proxy suspends its activities. If 
-'RESUME' is received, it goes on. If 'TERMINATE' is received, it terminates
-smoothly. If 'STATISTICS' is received, the proxy will reply on the control socket
-sending a multipart message with 8 frames, each with an unsigned integer 64-bit
-wide that provide in the following order:
- - number of messages received by the frontend socket
- - number of bytes received by the frontend socket
- - number of messages sent out the frontend socket
- - number of bytes sent out the frontend socket
- - number of messages received by the backend socket
- - number of bytes received by the backend socket
- - number of messages sent out the backend socket
- - number of bytes sent out the backend socket
-
-At start, the proxy runs normally as if zmq_proxy was used.
-
-If the control socket is NULL, the function behave exactly as if linkzmq:zmq_proxy[3]
-had been called.
-
-
-Refer to linkzmq:zmq_socket[3] for a description of the available socket types.
-Refer to linkzmq:zmq_proxy[3] for a description of the zmq_proxy.
-
-EXAMPLE USAGE
--------------
-cf zmq_proxy
-
-RETURN VALUE
-------------
-The _zmq_proxy_steerable()_ function returns 0 if TERMINATE is sent to its
-control socket. Otherwise, it returns `-1` and 'errno' set to *ETERM* or
-*EINTR* (the 0MQ 'context' associated with either of the specified sockets was
-terminated) or *EFAULT* (the provided 'frontend' or 'backend' was invalid).
-
-
-EXAMPLE
--------
-.Creating a shared queue proxy
-----
-//  Create frontend, backend and control sockets
-void *frontend = zmq_socket (context, ZMQ_ROUTER);
-assert (frontend);
-void *backend = zmq_socket (context, ZMQ_DEALER);
-assert (backend);
-void *control = zmq_socket (context, ZMQ_SUB);
-assert (control);
-
-//  Bind sockets to TCP ports
-assert (zmq_bind (frontend, "tcp://*:5555") == 0);
-assert (zmq_bind (backend, "tcp://*:5556") == 0);
-assert (zmq_connect (control, "tcp://*:5557") == 0);
-
-// Subscribe to the control socket since we have chosen SUB here
-assert (zmq_setsockopt (control, ZMQ_SUBSCRIBE, "", 0));
-
-//  Start the queue proxy, which runs until ETERM or "TERMINATE" 
-//  received on the control socket
-zmq_proxy_steerable (frontend, backend, NULL, control);
-----
-.Set up a controller in another node, process or whatever
-----
-void *control = zmq_socket (context, ZMQ_PUB);
-assert (control);
-assert (zmq_bind (control, "tcp://*:5557") == 0);
-
-// pause the proxy
-assert (zmq_send (control, "PAUSE", 5, 0) == 0);
-
-// resume the proxy
-assert (zmq_send (control, "RESUME", 6, 0) == 0);
-
-// terminate the proxy
-assert (zmq_send (control, "TERMINATE", 9, 0) == 0);
-
-// check statistics
-assert (zmq_send (control, "STATISTICS", 10, 0) == 0);
-zmq_msg_t stats_msg;
-
-while (1) {
-    assert (zmq_msg_init (&stats_msg) == 0);
-    assert (zmq_recvmsg (control, &stats_msg, 0) == sizeof (uint64_t));
-    assert (rc == sizeof (uint64_t));
-    printf ("Stat: %lu\n", *(unsigned long int *)zmq_msg_data (&stats_msg));
-    if (!zmq_msg_get (&stats_msg, ZMQ_MORE))
-        break;
-    assert (zmq_msg_close (&stats_msg) == 0);
-}
-assert (zmq_msg_close (&stats_msg) == 0);
----
+The _zmq_proxy_steerable()_ function is an empty stub that only returns an
+*EOPNOTSUPP* error, as the author did not provide a relicense agreement for
+the Mozilla Public License v2 relicense of libzmq.
 
 
 SEE ALSO