Multiple improvements to sniffer.py:
* Remove ambiguity of configuring recvif, it must be now explicitly specified.
* Don't catch exceptions around creating the sniffer, let it properly
fail and display the whole stack trace.
* Count correct packets so that duplicates can be found.
MFC after: 1 week
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D38120
Since 2.4.5 scapy started issuing warnings about a few different
configurations during our tests. These are harmless, but they generate
stderr output, which upsets atf_check.
Configure scapy to only log critical errors (and thus not warnings) to
fix these tests.
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")