BEFORE: First packet sent by client-side plugin (generated by Windows
function InitializeSecurityContext()) could be longer than 255 bytes 
violating the limitation imposed by authentication protocol.

AFTER: Handshake protocol is  changed so that if first client's reply is 
longer than 254 bytes then  it is be sent in 2 parts. However, for replies
shorter than 255 bytes nothing changes.

ADDITIONAL CHANGES: 
- The generic packet processing loop  (Handshake::packet_processing_loop) 
has been refactored. Communication with the peer has been abstracted
into virtual methods read/write_packet() which are implemented in client 
and server and transparently do the required splitting and gluing of packets.
- Make it possible to optionally use dbug library in the plugin.
- Add code for testing splitting of long first client reply.