The new protocol adds support for protocol version and  exchanges the
socket type, so that the library can reject a connection when the
sockets do not match.

The protocol was designed so that it's possible to detect and fully
support ZTP/1.0 peers.

When a new connection is set up, peers exchange greeting messages. The
greeting message encodes both the protocol verion and the socket type.
The format of the greeting message is as follows:

    greeting    = tag1, adaptation, tag2, version, length, socket_type
    tag1        = BYTE / 0xff
    adaptation  = 8 BYTES
    tag2        = BYTE / 0x7f
    version     = BYTE / 1
    length      = BYTE / 1
    socket_type = BYTE

The protocol does not define the value of adaptation field.

When interoperability with ZTP/1.0 peers is required, the adaptaion
encodes, in network byte order, the length of identity message increased
by 1. When adaptaion consists of eight zeros, the current
implementatatio of 0MQ 2.x closes the connection.

This patch supports both ZTP/1.0 and new protocol.