Fragmented IPv4, PayloadLen(), and ipv6 Len() fix.
- Don't decode non-first fragmented IP datagrams.
- Introducing 'PayloadLen()' which returns IPv[46] payload lengths, considering
  IPv6 extended headers.
- IPv6 Len() now returns the length of the _whole_ packet. the '.Length'
  attribute doesn't count the 40 fixed bytes from the header.