Fixes related to pre-upgrade orphan check.

checks.py: - Clear pre_upgrade_orphans when re-running the check. - When running the check, always reset self.info, not just if there are packages to add to it. - Set RESULT_SUCCESS if the check passes without any orphans found - otherwise this may remain RESULT_INFO when running the check more than once. mintupgrade.py: - Always run_next_check() on success, not just if check is in self.checks. Checks with RESULT_INFO are removed after their first run.