Let's bring the credentials into a better order, in order of relevance.
Also, let's clarify what the generic LUKS PIN is about.
Finally, list the credentials in system-credentials(7) too, after all
people might want to unlock a disk with this via SMBIOS Type 11 or so.
cryptsetup reads a bunch of credentials now but we don't pass import
those in any service units yet. Let's pass through all cryptsetup
prefixed credentials to the systemd-cryptsetup@root instance.