Add abstract namespace support for IPC sockets on Linux.

See issue 567.

Converts an initial strudel or "at sign" (@) in the Unix socket path to
a NULL character ('\0') indicating that the socket uses the abstract
namespace instead of the filesystem namespace.  For instance, binding a
socket to 'ipc://@/tmp/tester' will not create a file associated with
the socket whereas binding to 'ipc:///tmp/tester' will create the file
/tmp/tester.  See issue 567 for more information.

bugfixes for mingw compilation

replace macro constants with enum types for user facing constants

Stop curve handshake when cookie box verification fails

Clarified that zmq.h constants are part of the public contract

fix minor typo in zmq_ctx_get doc example

Packaging for zmq_curve_keypair function

* Added new man page for this
* Added test case, in tests/test_security_curve.cpp
* Noted in zmq_utils.h that these methods are documented

Expose zmq_curve_keypair via zmq_utils

asserts aren't appropriate for checking user input.

config for libsodium in alternate path, using CPP flags for both C and C...

include missing platform.hpp in curve_keygen

it was excluded, so HAVE_SODIUM would never be defined,
leading to curve_keygen always reporting "recompile with libsodium"

they are used by curve_keygen, but not available

This pull requests fixes building on SmartOS and possibly other Solaris-like systems

Updated CMake project for test_issue_566

Added test case for issue 566

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

Build/test errors on OS/X with clang++

Call flush after writing the identity message

Updated libzmq CURVE to track RFC 27