Initial support for guided disk encryption

Initial support for guided disk encryption

One new question to cover the most common use case, such that manual setup
in (S)hell or '!' prior to install is no longer required:

    Encrypt the root disk? (disk, 'no' or '?' for details) [no] ?

    Create a passphrase protected CRYPTO softraid volume to be used as root disk.

    Available disks are: sd0.
    Encrypt the root disk? (disk, 'no' or '?' for details) [no]

Use of keydisk or different disciplines are not covered.
Only asked in interactive installations;  no autoinstall(8) or upgrades.
Only reachable on i386, amd64, sparc64 and riscv64 for now (arm64 WIP).

Tested by cheloha naddy and a few users
Feedback from cheloha deraadt claudio
OK cheloha
"get it in now" deraadt