Arduino Pro and Arduino Pro Mini, pin 13 initialized as output

Hi,
pin 13 is unexpecteadly initialized as output. This code shows that the on board led on pin 13 blinks, without the pin being set as output in the code.

```
void setup(){}

void loop()
{
delay(1000);
digitalWrite(13, HIGH);
delay(1000);
digitalWrite(13, LOW);
}
```

Reproduce the behavior on an Arduino Pro or Pro Mini with the supplied setup/loop sketch, then trace how the AVR core initializes pin 13 before the sketch runs. Done means pin 13 is not initialized as an output unexpectedly, and the supplied sketch no longer blinks the onboard LED without explicitly setting the pin mode.

Written by the indexing model from the issue text.