Support use with systemd-gpt-auto-generator on LUKS devices

[Cornelicorn]

Currently, LUKS containers are being unlocked to luks-$UUID, which produces predictable names. This fails though, if you want to use a system without an fstab using systemd-gpt-auto-generator. This checks the GPT partition type GUID and expects LUKS mappers to be unlocked to the respective filesystem name, e.g. root for /.

It would be nice if clevis would support this for the defined partition type GUIDs and fallback to luks-$UUID otherwise.

This would break compatability with older versions of clevis though, which is not wanted, I assume.

I guess an easy way to control this behaviour would be a cmdline option, if the compatability should not be broken.

I'm happy to implement this, but wanted to coordinate before I just write code that will be rejected in review :)