MEDIUM: improve config check return codes

MEDIUM: improve config check return codes

When checking a configuration file using "-c -f xxx", sometimes it is
reported that a config is valid while it will later fail (eg: no enabled
listener). Instead, let's improve the return values :
  - return 0 if config is 100% OK
  - return 1 if config has errors
  - return 2 if config is OK but no listener nor peer is enabled