Since we define the meaning of each exit code, we should be using that
ourselves.

Functional tests have been coded using literal values, and there may
be a good reason for that: proving that the API stands.  But there's a
big issue: we have not declared those codes as some kind of an
external API and guarantee stability. Also the subsequent changes in
this series actually change the meaning and values of those.
Signed-off-by: NCleber Rosa <crosa@redhat.com>