Problem: V3 protocol handler vulnerable to downgrade attacks

[hintjens]

It is easy to bypass the security mechanism in 4.1.0 and 4.0.5 by sending a ZMTP v2 or earlier header. The library accepts such connections without applying its security mechanism.

Solution: if security is defined on a socket, reject all V2 and earlier connections, unconditionally.

Fixed by #6cf120 and related commits.

[hintjens]

Backported to 4.0.x and 4.1.x.