Show the libpcap error when a BPF filter fails to compile

[eugen-goebel]

Description

compile_filter() reported Failed to compile filter expression <filter> (-1), which gave no indication of why the filter was rejected: a syntax error (such as the not arpand not port 22 typo from the issue), or a filter incompatible with the interface link-layer type (such as a wlan filter on an Ethernet link).

Both compilation paths now go through a pcap handle and, on failure, retrieve the underlying libpcap message via pcap_geterr(). The linktype path uses pcap_open_dead(), so it still needs neither an interface nor root privileges. The exception now reads, for example:

• Failed to compile filter expression 'wlan type mgt subtype deauth' (802.11 link-layer types supported only on 802.11)
• Failed to compile filter expression 'not arpand not port 22' (can't parse filter expression: syntax error)

A regression test is added in test/regression.uts.

fixes #4587

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.34%. Comparing base (14cc726) to head (e52dcb8).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/arch/common.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5022   +/-   ##
=======================================
  Coverage   80.34%   80.34%           
=======================================
  Files         386      386           
  Lines       96012    96015    +3     
=======================================
+ Hits        77137    77140    +3     
  Misses      18875    18875           

Files with missing lines	Coverage Δ
scapy/arch/common.py	72.72% <83.33%> (+2.88%)	⬆️

... and 2 files with indirect coverage changes

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[gpotter2]

Thanks for the PR !