Move d0 and d1 pins to d6 and d7 to free up rxd and txd (uart).
Reintroduce beep after startup, got lost in a previous commit.
Correct wrong DDR(C) value for I2C bus pins.
Correct pwm output. When the port should be off, don't just write 0x0000
in the OCR register, that won't work. Instead, disconnect the pins from
the timer and set them low explicitly.