This is a minor re-work of the entrypoint script. It isolates a
generate() method from the actual command-line mechanism.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[invalid_char() renamed to invalid_prefix_char()]
Signed-off-by: Markus Armbruster <armbru@redhat.com>