partitioned MP messages.

This moves certain properties into a second part of the buffer that isn't visible to the first with the intention of making a backwards compatible MP protocol that can have more efficient encoding in a second part that is after MsgLen.

Unfortunately this doesn't work because the code ensures that MsgLen in the header matches the received byte count.

src/MultiPlayer/mpmessages.hxx

@@ -55,6 +55,7 @@ const uint32_t PROTO_VER = 0x00010001;  // 1.1
 #define OLD_PROP_MSG_ID         5
 #define RESET_DATA_ID           6
 #define POS_DATA_ID             7
+#define MP_2017_DATA_ID         8
 
 // XDR demands 4 byte alignment, but some compilers use8 byte alignment
 // so it's safe to let the overall size of a network message be a 
@@ -70,7 +71,7 @@ struct T_MsgHdr {
     xdr_data_t  Version;                // Protocoll version
     xdr_data_t  MsgId;                  // Message identifier 
     xdr_data_t  MsgLen;                 // absolute length of message
-    xdr_data_t  ReplyAddress;           // (player's receiver address
+	xdr_data_t  MsgLen2;                // Used to be - ReplyAddress;           // (player's receiver address
     xdr_data_t  ReplyPort;              // player's receiver port
     char Callsign[MAX_CALLSIGN_LEN];    // Callsign used by the player
 };

src/MultiPlayer/multiplaymgr.hxx

@@ -101,6 +101,9 @@ private:
   typedef std::map<unsigned int, SGSharedPtr<SGPropertyNode> > PropertyMap;
   PropertyMap mPropertyMap;
 
+  typedef std::map<unsigned int, const struct IdPropertyList*> PropertyDefinitionMap;
+  PropertyDefinitionMap mPropertyDefinition;
+
   bool mPropertiesChanged;
   MPPropertyListener* mListener;