Add support for SOCKS proxies

This is still raw and experimental.
To connect through a SOCKS proxy, set ZMQ_SOCKS_PROXY socket option on
socket before issuing a connect call, e.g.:

    zmq_setsockopt (s, ZMQ_SOCKS_PROXY,
        "127.0.0.1:22222", strlen ("127.0.0.1:22222"));
    zmq_connect (s, "tcp://127.0.0.1:5555");

Known limitations:
- only SOCKS version 5 supported
- authentication not supported
- new option is still undocumented

src/stream_engine.hpp

@@ -92,16 +92,6 @@ namespace zmq
         //  Detects the protocol used by the peer.
         bool handshake ();
 
-        //  Writes data to the socket. Returns the number of bytes actually
-        //  written (even zero is to be considered to be a success). In case
-        //  of error or orderly shutdown by the other peer -1 is returned.
-        int write (const void *data_, size_t size_);
-
-        //  Reads data from the socket (up to 'size' bytes).
-        //  Returns the number of bytes actually read or -1 on error.
-        //  Zero indicates the peer has closed the connection.
-        int read (void *data_, size_t size_);
-
         int identity_msg (msg_t *msg_);
         int process_identity_msg (msg_t *msg_);