flexible auth key backup and restore

[flihp]

The oks hsm restore command was developed as a way to ensure our backup key export & shamir splitting scheme could be restored. This needs to be integrated into the larger flow of a ceremony that restores the backup key and then does something with them.

[flihp]

Consider using the verifier produced when the key is initially split to validate shares as they're entered. This could save us some pain and suffering if shares are entered incorrectly.