scubainit: Rename use_root to should_change_user
scubainit: Rename use_root to should_change_user

This restores the polarity to the way it was before b83f5ff0, but with a
different name (since there is no "scuba user" now).

JonathonReinhart/scuba,JonathonReinhart/scuba,JonathonReinhart/scuba