Decode IPv6 Jumbograms

[guedou]

This PR is a bit tricky as some parsing must be in IPv6.extract_padding() to retrieve the payload length from the payload itself.

[guedou]

Ready to be merged. The failing AppVeyor is related to nmap.

[codecov-io]

Codecov Report

Merging #1196 into master will increase coverage by 0.05%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1196      +/-   ##
==========================================
+ Coverage   84.04%   84.09%   +0.05%     
==========================================
  Files         159      159              
  Lines       38211    38228      +17     
==========================================
+ Hits        32114    32148      +34     
+ Misses       6097     6080      -17

Impacted Files	Coverage Δ
scapy/layers/inet6.py	85.78% <90%> (+0.02%)	⬆️
scapy/fields.py	90.7% <0%> (+0.17%)	⬆️
scapy/config.py	85.39% <0%> (+0.27%)	⬆️
scapy/asn1/ber.py	82.28% <0%> (+0.28%)	⬆️
scapy/layers/tls/basefields.py	80.27% <0%> (+0.68%)	⬆️
scapy/layers/tls/record.py	92.08% <0%> (+0.87%)	⬆️
scapy/layers/tls/record_sslv2.py	88.82% <0%> (+1.67%)	⬆️
scapy/layers/tls/keyexchange.py	84.76% <0%> (+1.71%)	⬆️