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

NULL mechanism sends ERROR command rather than READY command when ZAP
handler rejects the ZAP request (status code != "200"). The body of
ERROR command contains the status code as returned by ZAP handler.

We use metadata_t directly. No need for generic interface now.

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.

- additional messages to help people debugging security errors

Adjust test cases to connection notification.
Increase error checking in test cases.

When a ZMQ_STREAM socket connection is broken (intentionally, via `shutdown()`
or accidentally via client crash or network failure), there is no way for the
application to dertermine that it should drop per-connection data (such as
buffers).

This contribution makes sure the application receives a 0-length message to
notify it that the connection has been broken.  This is symmetric with the
process of closing the connection from within the application (where the
application sends a 0-length message to tell ZeroMQ to close the connection).

Conflicts:
	CMakeLists.txt

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.

This reverts commit f27eb67e, reversing
changes made to a3ae0d4c.

https://zeromq.jira.com/browse/LIBZMQ-576

Conflicts:
	src/stream_engine.cpp

Conflicts were around additional defaults to the constructor after the
'terminating' default.  The additional defaults were left alone, and
the 'terminating' default was removed.

- This seems redundant; is there a use case for NOT providing
  the IPC credentials to the ZAP authenticator?

- More, why is IPC authentication done via libzmq instead of ZAP?
  Is it because we're missing the transport type on the ZAP request?

Another take on LIBZMQ-568 to allow filtering IPC connections, this time
using ZAP.  This change is backward compatible.  If the
ZMQ_ZAP_IPC_CREDS option is set, the user, group, and process IDs of the
peer process are appended to the address (separated by colons) of a ZAP
request; otherwise, nothing changes.  See LIBZMQ-568 and zmq_setsockopt
documentation for more information.

* Tests dealer-to-router connection 100 times
* This was failing in ZMQ v4.0.0 RC1

* Command names changed from null terminated to length-specified
* Command frames use the correct flag (bit 2)
* test_stream acts as test case for command frames
* Some code cleanups

The window scale option carried in SYN segment is computed from socket's
receive buffer size. So we need to set this buffer size before calling
connect or bind.