When rb-site was first written, back in pre-1.0, it supported a

console UI and a GTK+ UI, to help get set up on graphical Linux

installs. We've long since removed the graphical UI, but left the base

foundation of it around, with no peer.
This change strips out the base class, leaving only ConsoleUI around.

In the process, it documents it and improves a little of the

functionality to help with reuse.
This is a precursor to moving this file and splitting up some of the

responsibilities so that some of the functionality can be used by

rbext, and so rb-site can be simplified a bit.

Tested creation and upgrades of sites. All functionality worked as

expected.