Fix sniff timeout when libpcap select returns readable with no packets

[T3pp31]

Summary

• When sniff(..., timeout=N) uses a libpcap socket, select may report the capture fd as readable even when no BPF-matched packets are available; a blocking pcap_next_ex in recv() could then stall past the sniff timeout (same class as Timeout doesn't work when no sniffing results with L2pcapListenSocket #74).
• Timed sniff loops now call nonblock_recv() when the socket supports it, via a nonblock flag on session recv().
• Adds a mock regression test that simulates libpcap always returning the socket from select while nonblock_recv() yields no packet.

Fixes #4590

Test plan

• New regression test: sniff timeout with select-ready but empty pcap socket (#4590) in test/regression.uts
• Manual verification: mock socket completes within ~1s with timeout=1, blocking recv() never called
• CI (UTscapy on Linux with libpcap)

Made with Cursor

[gpotter2]

Hi & thanks for the PR !

It has great performance implications to use a non blocking read instead of a blocking one. I am therefore not sure that this is the proper way of tackling the issue.