Fold disk crypto question '?' text into prompt

Fold disk crypto question '?' text into prompt

Since this question moved after the root disk one and '?' stopped listing disks,
hoist the implementation details in order to drop the custom answer and reuse
existing ask_yn(), thus

	Encrypt the root disk? (yes, no or '?' for details) [no] ?
	Create a passphrase protected CRYPTO softraid volume to be used as root disk.
	Encrypt the root disk? (yes, no or '?' for details) [no]
becomes
	Encrypt the root disk? (passphrase CRYPTO softraid) [no]

Prodded by afresh1