internet: fix UDP header checksum
RFC 768: If the computed checksum is zero, it is transmitted as all ones
This MR fixes that, and the decoding as well.
Thanks the Almighty that I'm pedantic and I triple check calculations. It turns out that on Rx you don't have to do any math, because it's not needed.
I added a LONG note on why it's not needed, for future reference.
Edited by Tommaso Pecorella