Add prompt.auto-init-on-unresolved-project option

Some users prefer not to be interrupted by such prompts, so pave the way
to creating options to disable those that might get in the way.

Follow the example of the advice.* options 'git-config', and create a
namespace for these UI options grouped by behaviour, rather than an
over-reaching 'ui.*' namespace. In later work perhaps we'll also add
'advice.*' options.