Introduce xMessageBufferNextLengthBytes() and tests for the same.

Add call to traceTASK_SWITCHED_IN() in vTaskStartScheduler() so trace tools can see the first task to run.

FreeRTOS/Source/tasks.c

@@ -1994,6 +1994,8 @@ BaseType_t xReturn;
 		FreeRTOSConfig.h file. */
 		portCONFIGURE_TIMER_FOR_RUN_TIME_STATS();
 
+		traceTASK_SWITCHED_IN();
+
 		/* Setting up the timer tick is hardware specific and thus in the
 		portable interface. */
 		if( xPortStartScheduler() != pdFALSE )