luks2: Use more intuitive object name instead of json index in user messages

luks2: Use more intuitive object name instead of json index in user messages

Use the object name in the json array rather than the 0 based index in the
json array for keyslots, segments, and digests. This is less confusing for
the end user. For example, say you have a LUKS2 device with a key in slot 1
and slot 4. When using the password for slot 4 to unlock the device, the
messages using the index of the keyslot will mention keyslot 1 (its a
zero-based index). Furthermore, with this change the keyslot number will
align with the number used to reference the keyslot when using the
--key-slot argument to cryptsetup.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>