Linux - check if auxdata is available

[guedou]

This PR fixes #2447 and makes Scapy works on Linux kernel < 2.6.21 with Python3.

There are two issues:

1. no auxdata support before Linux 2.6.21
2. Python3 recvmsg() returns data when auxdata are not supported

[codecov]

Codecov Report

Merging #2460 into master will increase coverage by 0.75%.
The diff coverage is 4.34%.

@@            Coverage Diff             @@
##           master    #2460      +/-   ##
==========================================
+ Coverage    86.7%   87.46%   +0.75%     
==========================================
  Files         243      242       -1     
  Lines       50802    49949     -853     
==========================================
- Hits        44047    43686     -361     
+ Misses       6755     6263     -492

Impacted Files	Coverage Δ
scapy/arch/linux.py	74.75% <0%> (+20.61%)	⬆️
scapy/supersocket.py	67.09% <6.25%> (+15.77%)	⬆️
scapy/contrib/cansocket.py	58.33% <0%> (-12.51%)	⬇️
scapy/contrib/scada/iec104/__init__.py	92.79% <0%> (-7.21%)	⬇️
scapy/autorun.py	72.85% <0%> (-5.66%)	⬇️
scapy/layers/tls/automaton_srv.py	73.18% <0%> (-4.72%)	⬇️
scapy/layers/tls/automaton_cli.py	79.62% <0%> (-4.02%)	⬇️
scapy/contrib/isotp.py	78.69% <0%> (-3.88%)	⬇️
scapy/layers/tftp.py	88.27% <0%> (-1.39%)	⬇️
scapy/asn1/ber.py	82.81% <0%> (-0.88%)	⬇️
... and 73 more

[gpotter2]

Thanks for fixing this