homework: Accept volume key from keyring

homework: Accept volume key from keyring

This bypasses authentication (i.e. user_record_authenticate) if the
volume key was loaded from the keyring and no secret section is
provided.

This also changes Update() and Resize() to always try and load the
volume key from the keyring. This makes the secret section optional for
these methods while still letting them function (as long as the home
area is active)