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