Add pull-up/pull-down support for GPIO

[mrmorphic]

This might come under "unsafe" module territory, as kernel drivers don't appear to support pud configuration. Need to test if this works reliably, and provide a way for consumer to indicate unsafe ops are OK. GPIO module is currently common to both rpi and BBB, but memory-mapped I/O will be very different, so perhaps some way to add extensions to a module. Otherwise create alternate modules, but alot of work. OTOH, if all GPIO is memory mapped, it will be extremely fast,