As per suggestion by Pieter Hintjens. Also update wording in
zmq_ctx_set manual a bit.

This reverts commit 9ae6a91f.

This reverts commit bc7441f5.

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.

- renamed to ZMQ_CONNECT_RID
- fixed whitespace malformating around previous patch
- renamamed next_peer_id to next_rid in preparation for
  larger rename of IDENTITY to ROUTING_ID

Note: ZMQ_CONNECT_RID has no test case and no entry in the man
page, as yet.

Fixed compile issue with missing member of socket_base.  Changed ZMQ_NEXT_IDENTITY to ZMQ_NEXT_CONNECT_PEER_ID.

Fixed case where ZMQ_NEXT_CONNECT_PEER_ID is used in ROUTER, and ROUTER does not read the identity message from the connected pipe.

Both STREAM and ROUTER sockets suffer from a naming problem on outbound connections. While these connections can be created, they can't be immediately used. Traffic must be received before it can be sent. This prevents practical, minimal usage of STREAM or ROUTER as a true N fan in/out socket.

This change simply provides the user with a socket option that sets a user defined name of the next outbound connection:

zmq_setsockopt(routerSock,ZMQ_NEXT_IDENTITY,"myname",6);
if(0 > zmq_connect(routerSock,"tcp://127.0.0.1:1234")) return 1;
ret = zmq_send(routerSock,"myname",6,ZMQ_SNDMORE);
zmq_send(routerSock,b.mem,b.used,0);

In this example, the socket is immediately given the name "myname", and is capable of immediately sending traffic.

This approach is more effective in three ways:
1) It prevents all sorts of malicious peer naming attacks that can cause undefined behavior in existing ROUTER connections. (Two connections are made that both transmit the same name to the ROUTER, the ROUTER behavior is undefined)
2) It allows immediate control of connections made to external parties for STREAM sockets. Something that is not possible right now. Before an outbound connection had no name for STREAM or ROUTER sockets because outbound connections cannot be sent to without first receiving traffic.
3) It is simpler and more general than expecting two ROUTER sockets to handshake on assigned connection names. Plus it allows inline sending to new connections on ROUTER.

Also moved the fd field from message content to message itself

This patch allows client applications to retrieve the remote endpoint from a message that originated from a tcp socket

- 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.

Adds ZMQ_IPC_ACCEPT_FILTER_{PID,UID,GID} socket options to add processs,
user, and group IDs to the IPC filter sets. Fixes LIBZMQ-568.

Shorten ZMQ_ROUTER_REASSIGN_IDENTITIES to ZMQ_ROUTER_HANDOVER. Also, added a test for the HANDOVER functionality.

This reverts commit d4bb6d69.

This reverts commit e7804c50.

* ZMQ_REQ_STRICT was negative option (default 1) which goes against
  the standard, where defaults are zero. I renamed this to
  ZMQ_REQ_RELAXED.

* ZMQ_REQ_REQUEST_IDS felt clumsy and describes the technical solution
  rather than the problem/requirement. I changed to ZMQ_REQ_CORRELATE
  which seems more explicit.

This reverts commit bfd472f9.

This reverts commit 1206f457.

* Removed redundant Z85 code and include files from project
* Simplified use of headers in test cases (now they all just use testutil.hpp)
* Export zmq_z85_encode() and zmq_z85_decode() in API
* Added man pages for these two functions

* This is passed to the ZAP handler in the 'domain' field

* If not set, or empty, then NULL security does not call the ZAP handler

* This resolves the phantom ZAP request syndrome seen with sockets where
  security was never intended (e.g. in test cases)

* This means if you install a ZAP handler, it will not get any requests
  for new connections until you take some explicit action, which can be
  setting a username/password for PLAIN, a key for CURVE, or the domain
  for NULL.