Add feature to briefly run the cooling fan at full speed when it first starts.

The cooling fan on my printer will only start with a very high PWM value (~95%).  However, once it does start I can reliably run it at a wide range of PWM values (15% to 100%).  To make this work reliably for the full range of PWM values, I modified the Marlin firmware to briefly run the fan at full speed when it first starts spinning.