spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

netpfil tests: improve sniffer.py

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

tests: Only log critical errors from scapy

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")

pf tests: Explicitly ask for python3

If we install the scapy package (which we do list as a dependency) we
don't automatically install python (but we do have python3).

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC (“Netgate”’)

netpfil tests: Add missing copyright & license statements

Upgrade (scapy) py2 tests to work on py3.

In order to move python2 out of the test framework to avoid py2 vs. py3
confusions upgrade the remaining test cases using scapy to work with py3.
That means only one version of scapy needs to be installed in the CI system.
It also gives a path forward for testing i386 issues observed in the CI
system with some of these tests.

Fixes are:
- Use default python from environment (which is 3.x these days).
- properly ident some lines as common for the rest of the file to avoid
errors.
- cast the calculated offset to an int as the division result is considered
a float which is not accepted input.
- when comparing payload to a magic number make sure we always add the
payload properly to the packet and do not try to compare string in
the result but convert the data payload back into an integer.
- fix print formating.

Discussed with: lwhsu, kp (taking it off his todo :)
MFC after: 2 weeks

pf tests: Fix accidental duplication of content

Some files got their contented duplicated in r345409. Some mistakes where
fixed in r345430. The only file that was left with a duplicated content was
CVE-2019-5598.py.

Reviewed by: kp
Approved by: src (kp)
Differential Revision: https://reviews.freebsd.org/D21267

pf tests: Test CVE-2019-5598

Verify that pf correctly drops inconsistent ICMP packets (i.e. where the
IP src/dst do not match the IP src/dst in the ICMP packet.