zmq_atomic_counter_dec returned a 'bool' value, yet this isn't
defined by standard, so causes compile errors in upstream code.

Solution: return an int that can be safely converted to bool if
needed by bindings.

Problem: need atomic reference counting in several projects

Solution: as libzmq already provides this across all platforms,
expose an atomic counter API. I've not wrapped atomic pointers,
though someone who needs this may want to do so.

Problem: very hard to debug security mechanism mismatch

E.g. when server is not configured, and client tries PLAIN security,
there is no hint of why this does not work.

Solution: add debugging output for this case. Note that the various
debugging outputs for security failures should probably be sent to
an inproc monitor of some kind.

Problem: zmq_epgm is duplicate of zmq_pgm

There is zero benefit to this, and some confusion due to the special
case nature of this man page.

Solution: delete the zmq_epgm man page.

Fix #1274

reject old ZMTP connections if auth enabled

use explicit ZMTP/1.0 anonymous greeting
rather than HTTP request that just happened to work

- add ws2tcpip.h
- alias close->closesocket
- increment port in sec_null test

checks mechanism != NULL, or NULL + non-empty zap_domain

auth mechanisms were only enabled when ZMTP handshake
is latest version, meaning that connections from old sockets
would skip authentication altogether

fails on all auth mechanisms

Manual and welcome documentation

remove debug printf in test_xpub_welcome_msg

Manual and welcome

Fix issue #1257

Fix test_filter_ipc for cleared supplementary groups

This should fix part of [https://github.com/zeromq/libzmq/issues/1129].

Problem: linger values other than -1 or 0 are unsafe

Solution: set defaults back to infinity, and add new context
option, ZMQ_BLOCKY that the user can set to false to get a
less surprising behavior on context termination. Eg.

    zmq_ctx_set (ctx, ZMQ_BLOCKY, false);

Fix busy-polling when reaching the rate limit

use more conservative 30 second default timeout for LINGER

Regression in #1248 can cause lost messages at exit.

Problem: return code of sodium_init() is not checked.

There are two todo comments in curve_client.cpp and curve_server.cpp that suggest
checking the return code of sodium_init() call. sodium_init() returns -1 on error,
0 on success and 1 if it has been called before and is already initalized:
https://github.com/jedisct1/libsodium/blob/master/src/libsodium/sodium/core.c

Use consistent spelling, fix typos.

behavior -> behaviour
initialize -> initialse
initialization -> initialisation