After an extended absence (quit job, set up company, get girlfriend, get
contract, start contract, work too many hours per day) I'm back working
on the DocBook conversion :-)

Create two new entities, prompt.root and prompt.user. Use these where the
user is shown an OS prompt, to indicate whether they should be a normal
user or do it as root.

Everything else that looks like a prompt (e.g., C:\> which occurs here
and there) is also marked up as <prompt>.