TCP: put TCP-AO option decoding right

TCP: put TCP-AO option decoding right

As it was correctly pointed out in GitHub issue #516, the TCPOPT_TCPAO
(formerly TCPOPT_AUTH) case had an issue with option length processing,
though without significant consequences thanks to a check elsewhere.
Besides that, the old code (introduced in 2005) decoded a structure
similar to a proposed encoding variant of the early (first published in
2007) revisions of the Internet-Draft but different from the encoding
of RFC 5925 (published in 2010). These issues are now addressed and the
TCP option renamed to TCP-AO.