BUG#11879051: FIRST REPLY LENGTH LIMIT (255) CAN BE VIOLATED

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.

libmysql/authentication_win/handshake.h

@@ -100,7 +100,7 @@ public:
   Handshake(const char *ssp, side_t side);
   virtual ~Handshake();
 
-  int Handshake::packet_processing_loop(Connection &con);
+  int Handshake::packet_processing_loop();
 
   bool virtual is_complete() const
   {
@@ -126,6 +126,13 @@ protected:
   /// Stores attributes of the created security context.
   ULONG  m_atts;
 
+  /**
+    Round of the handshake (starting from round 1). One round
+    consist of reading packet from the other side, processing it and
+    optionally sending a reply (see @c packet_processing_loop()).
+  */
+  unsigned int m_round;
+
   /// If non-zero, stores error code of the last failed operation.
   int  m_error;
 
@@ -152,7 +159,13 @@ protected:
     @return A blob with data to be sent to the other end or null blob if
     no more data needs to be exchanged.
   */
-  virtual Blob process_data(const Blob &data)= 0;
+  virtual Blob process_data(const Blob &data) =0;
+
+  /// Read packet from the other end.
+  virtual Blob read_packet()  =0;
+
+  /// Write packet to the other end.
+  virtual int  write_packet(Blob &data) =0;
 
 #ifndef DBUG_OFF