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

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.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>

Documentation examples for zmq_msg_get and zmq_msg_more functions have an
incorrect call to zmq_msg_close function - with 'zmq_msg_t' as a parameter
despite 'zmq_msg_t *' is required, so it is impossible to compile these
examples properly.

Also for zmq_msg_get example - declaration of zmq_msg_t variable is added
(like it is done in other examples).