Comment 4 for bug 1332538

Has been discussed on IRC with Stéphane and Oliver. Stéphane and I believe the checking should be done at livefs build time by pre-populating the list of system users with a deterministic order.

This is a lot more reliable than having to fix things up in a boot hook (since in the event of problems we fail the image build, instead of failing the image upgrade), and also lets us fail the image build in the case of uncoordinated introduction of new system users.