core: dedicated function to check Via address with source ip

- received_via_test() as alternative to received_test() which looks also
  for rport and received parameters, confusing nat detection tests
nathelper: use dedicated function for testing via address against source ip

- reported by Igor Potjevlesch
- previous used functions was checking the presence of rport and
  received parameters, ending in a wrong result for nat testing
nat_traversal: use dedicated function for testing via address against source ip
tmx: proper indentation to match the module style