man: use ukify more in systemd-measure examples

man: use ukify more in systemd-measure examples

ukify supports signing with multiple keys, so show an example of this, and just
let ukify print the calls to systemd-measure that will be done.

This also does other small cleanups:
- Use more realistic names in examples
- Use $ as the prompt for commands that don't require root (most don't).
  Once we switch to operations that don't require a TPM, we should be able to get
  rid of the remaining calls that require root.
- Ellipsize or linebreak various parts
- Use --uname. We warn if it is not specified and we have to do autodetection, so
  let's nudge people towards including it rather than not.

Follow-up for e069c57f0616d39363d36ac7f9c3e6ec8be01ab1.