Reverted use of enums for zmq.h constants (too painful for binding authors)

This reverts commit d4bb6d69.

This reverts commit e7804c50.

Updated NEWS for 4.0.1 release

Typo in type name "emTPy_slots_t"

https://zeromq.jira.com/browse/LIBZMQ-565

Add documentation on Linux ipc abstract namespace support and a bug fix.

Abstract socket pathnames must have a NULL character in the first
position, but the second character must also be checked to differentiate
an abstract name from the empty string.  The address length must also
indicate the length of the pathname because the kernel uses the entire
address as the name, including NULL characters.  ZMQ uses
NULL-terminated strings for the address, so the abstract address length
is the length of the string following the initial NULL byte plus 3; two
bytes for the address family and one for the initial NULL character.

It is safe to "free(NULL)" and "delete NULL"

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"