creds-util: add an explicit 128bit ID for identifying "automatic" key determination

creds-util: add an explicit 128bit ID for identifying "automatic" key determination

Previously, when encrypting creds you could pick which key to use for
this via a 128bit ID identifying the key type, and use an all zero ID
for rquesting automatic mode.

Let's change this to use an explicitly picked 128bit ID for automatic
mode, i.e. something other than all zeros. This is in preparation for
adding one further automatic mode with slightly different semantics.

no change in behaviour.

Note that the new 128bit id is never written to disk but only used
internally to indicate a specific case.