Rename uid0 to run0

Rename uid0 to run0

Naming is always a matter of preference, and the old name would certainly work,
but I think the new one has the following advantages:
- A verb is better than a noun.
- The name more similar to "the competition", i.e. 'sudo', 'pkexec', 'runas',
  'doas', which generally include an action verb.
- The connection between 'systemd-run' and 'run0' is more obvious.

There has been no release yet with the old name, so we can rename without
caring for backwards compatibility.