A few minor changes for gopacket usage.

1) gofmt formatting
2) use gopacket.SerializeLayers to do simpler serialization
3) add FixLengths to serialize options, which fixes TCP Length
4) remove IPv4 fragment offset, since this is not a fragmented packet
5) maybe some other stuff I forgot?

BTW, if you want to see just the changes that do something, you can commit a gofmt first, then pull this in ;)