Ask for disk crypto after root disk question

Ask for disk crypto after root disk question

    Encrypt the root disk? (disk, no or '?' for details) [no]
    Which disk is the root disk? ('?' for details) [sdN]
becomes
    Which disk is the root disk? ('?' for details) [sd0]
    Encrypt the root disk? (yes, no or '?' for details) [no]

so that answering 'sd0' at this point during installation behaves the same
as before the new question:  specifying the root disk.

Users no longer deal with two (softraid chunk sd0, root disk sd1), but just
one disk as before, while sdN are dealt with transparently.

Code also gets simpler and should enable more improvements soon.

Feedback OK afresh1