Add interface for easy protocol implementation

This commit adds a protocol interface to the server, its purpose is to manage TCP connections and the various third-party protocols.

More specifically, ProtoHandleConnection() takes care of exchanging the packets between the local and remote endpoint; the protocol implementation only has to parse them and act accordingly.

The interface knows which protocol is the connection for by calling IsPacketForMe(), a function implemented for each protocol.

src/Cedar/Cedar.c

@@ -1606,6 +1606,9 @@ void InitCedar()
 
 	// Initialize protocol module
 	InitProtocol();
+
+	// Initialize third-party protocol interface
+	ProtoInit();
 }
 
 // Free Cedar communication module
@@ -1616,6 +1619,9 @@ void FreeCedar()
 		return;
 	}
 
+	// Free third-party protocol interface
+	ProtoFree();
+
 	// Free protocol module
 	FreeProtocol();
 }