As libzmq is compiled with optional transports and security mechanisms,
there is no clean way for applications to determine what capabilities
are actually available in a given libzmq instance.

Solution: provide an API specifically for capability reporting. The
zmq_has () method is meant to be open ended. It accepts a string so
that we can add arbitrary capabilities without breaking existing
applications.

zmq.h also defines ZMQ_HAS_CAPABILITIES when this method is provided.

Applications that use ZMQ_IDENTITY can be trapped by the artificial
restriction on not using a binary zero as first byte. It's specially
nasty on random generated identities, e.g. UUIDs, as the chance of a
binary zero is low, so it will pass 255 out of 256 times.

Solution: remove the restriction.

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

Few examples:
tcp:://192.168.0.4:55555;192.168.0.254:1234
tcp:://192.168.0.4:0;192.168.0.254:1234
tcp:://eth2:55555;192.168.0.254:1234

Added modifiers reflect the following properties:
- zmq_msg_gets () does not mutate property parameter
- zmq_msg_gets () returns a pointer to memory the caller should not
  modify

- also removed deprecated man pages from Makefile
- they remain in repository for reference

Specifically:

* zmq_event_t should not be used internally in libzmq, it was
  meant to be an outward facing structure.

* In 4.x, zmq_event_t does not correspond to monitor events, so
  I removed the structure entirely.

* man page for zmq_socket_monitor is incomplete and the example
  code was particularly nasty.

* test_monitor.cpp needed rewriting, it was not clean.

Also the portion about how to close a connection does not only apply to
servers.

zmq_recv returns bytes count, not error code, so 'assert (rc==0)' behaved completely wrong

Issues adressed:
 - The actual data was never read from the socket, causing all even
   numbered loop iterations to fail
 - The socket variable was called server once

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.

- spaces instead of tabs for indentation
- fixed levels under EXAMPLE USAGE
- added to Makefile.am

Added assert on duplicate id in ROUTER and STREAM sockets.  Updated man to reflect this change.  Modified test case to match.

Updated man entries, and added behavior in case the user duplicates peer ids.  In this case the socket reverts to default behavior.

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

Also moved the fd field from message content to message itself

- 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?

- put into alphabetical order
- there was no consistency in previous ordering

Minor layout fix

Whitespace fix