Fix interface IP for point-to-point links

[0x-0ddc0de]

For IPv4 point-to-point (p2p) links, Linux rtnetlink provides two IP addresses for the link: IFA_LOCAL, which is the IP of the local interface, and IFA_ADDRESS, which is the peer's IP address. The existing parsing of the rtnetlink data uses IFA_ADDRESS as the local interface IP, regardless of the type of link it is, resulting in the peer IP being returned for p2p links. This manifests itself, for example, in conf.ifaces and get_if_addr().

This PR fixes the bug by detecting the existence of IFA_LOCAL and if present, using its value instead of IFA_ADDRESS. For non-p2p links, either both IFA_LOCAL and IFA_ADDRESS exist and are identical or just IFA_ADDRESS exists.

This fix was tested on local Linux machines connected to a remote VPN server, as a VPN tunnel forms a p2p link.

Fixes issue #4884

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.86%. Comparing base (e73137e) to head (965ec19).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4887   +/-   ##
=======================================
  Coverage   80.86%   80.86%           
=======================================
  Files         368      368           
  Lines       90271    90274    +3     
=======================================
+ Hits        72996    73002    +6     
+ Misses      17275    17272    -3     

Files with missing lines	Coverage Δ
scapy/arch/linux/rtnetlink.py	92.76% <100.00%> (+0.09%)	⬆️

... and 8 files with indirect coverage changes

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[gpotter2]

Thanks a lot for the PR !