reject old ZMTP connections if auth enabled

Auth mechanisms were only enabled when ZMTP handshake is the latest version, meaning that connections from old sockets would skip authentication altogether.

A test with a vanilla socket is added to all security tests. Prior to the patch, all security mechanisms failed the test, allowing messages to be delivered without authentication.

Adds `session->zap_enabled()`, which checks the security mechanism options for whether security is enabled. If ZAP is enabled, connections from ZMTP < 3 are prohibited.