Fix test_filter_ipc for cleared supplementary groups

This should fix part of [https://github.com/zeromq/libzmq/issues/1129].
Merge pull request #1260 from abbradar/master

Fix test_filter_ipc for cleared supplementary groups
Fix issue #1257
Merge pull request #1262 from hurtonm/master

Fix issue #1257
Add manual control over subscriptions to Pub
xpub welcome msg
Merge pull request #1267 from somdoron/manual_and_welcome

Manual and welcome
remove debug printf in test_xpub_welcome_msg
Merge pull request #1268 from somdoron/manual_and_welcome

remove debug printf in test_xpub_welcome_msg
documentation for welcome msg and xpub manual
add to authors
Merge pull request #1269 from somdoron/manual_and_welcome

Manual and welcome documentation
test unauthenticated messages from vanilla sockets

fails on all auth mechanisms
reject old ZMTP connections if auth enabled

auth mechanisms were only enabled when ZMTP handshake
is latest version, meaning that connections from old sockets
would skip authentication altogether
add session->zap_enabled()

checks mechanism != NULL, or NULL + non-empty zap_domain
allow vanilla socket security tests to run on Windows

- add ws2tcpip.h
- alias close->closesocket
- increment port in sec_null test
craft vanilla socket security test messages

use explicit ZMTP/1.0 anonymous greeting
rather than HTTP request that just happened to work
Merge pull request #1272 from minrk/security-old-zmtp

reject old ZMTP connections if auth enabled
Alter --with-libsodium default.
Merge pull request #1275 from pmienk/master

Fix #1274