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.

Fixes #811

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 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.

- corporate copyright statements sit in AUTHORS file

- 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?

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.

Add explicit check for primary group.

using -1 causes a warning on Windows platform because SOCKET is unsigned.

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.

Adds sets of process (Linux only), user, and group IDs for filtering
connections from peer processes over IPC transport.  If all of the
filter sets are empty, every connection is accepted.  Otherwise,
credentials for a connecting process are checked against the filter sets
and the connection is only accepted if a match is found.

This commit is part of LIBZMQ-568 and only adds the filter sets and
implements the filter in the IPC accept method.  The interface for
adding IDs to filter sets are included in a separate commit.

IPC accept filtering is supported only on Linux and OS X.

the size of the critical section is reduced by only entering the critical
section right before the bind().

Update signaler make_fdpair to set file descriptors to -1 when too many files are open using socketpair.

Fix socket creation above sistem limits for all 'other' OS not covered by eventfd, windows or vms; enhanced test to create sockets up to a bigger limit to really test hitting the OS limit