12 -> 13 upgrade and ZFS checkpoint

Hello.

TL;DR
Don't try to upgrade to 13 if you have a checkpoint on your boot pool.



I spent three hour yesterday trying to fix a production server which 
would not restart after an upgrade.
Even rolling back to previous version did not help.
After trying almost everything, I was able to see, among a load of fast 
scrolling cryptical messages, "can not read checkpoint data".
Turns out 13.1's loader.efi is not able to find a ZFS pool if it was 
checkpointed on 12.3.

Perhaps this is widely known, but I had never seen any mention of this.
Possibly this does not happen on non-UEFI systems.

I hope this message can save a lot of trouble to others.

  bye
	av.