initialize all pins to known states on startup

[ejovrh]

ATM the atmega2560 pins are in some obscure "default" initial state on startup (after setup() is complete).
for clarity's sake implement an initialization code block which shall set all pins and internal devices to known states:

• pins to high impedance
• turn off all needed devices
• turn on only needed devices

the initialization shall be binary values for the appropriate registers; using Arduino libraries for this is not worth the time.
document each setting via references to the datasheet.