Enable disk encryption in unattended installations

Enable disk encryption in unattended installations

Interactively keeps using bioctl(8)'s own prompt, in unattended mode
ask_passphrase() ensures non-empty responses or fails.

Unlike user passwords, autoinstall(8) only supports plaintext passphrases:
    Encrypt the root disk with a (p)assphrase or (k)eydisk = passphrase
    New passphrase = secret
Make sure to trust the install network or use a pre-configured key disk:
    Encrypt the root disk with a (p)assphrase or (k)eydisk = keydisk
    Which disk contains the key disk = sd2
    Which sd2 partition is the key disk = a

initial diff from Chris Narkiewicz
OK afresh1
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