Disconnected

This adds support for marking pins as "disconnected", which allows enabling pullups to save power.

To make it easy, a `pin.othersdisconnected` bitlash is added which switches all pins that do not have a pin mode yet to the new disconnected mode.

Also, a new "unset" mode is introduced as the default mode for pins. This allows explicitely marking some pins (which are connected something but currently unused) as "disabled", and then calling `pin.othersdisconnected` without changing those pins as well.

I'm not very happy with the `othersdisconnected` name yet, any better suggestions?

Also, this works for all pins that are present on the header and have Arduino pin numbers. There are a few more pins on the MCU that are floating, but they don't have a pin number, so we need some extra magic to enable pullups on those.