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

net tests: basic test case for bpf(4)'s ability to set vlan pcp

Use dhclient with its 'vlan-pcp' option to set a VLAN PCP value and
verify that it actually gets set.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31276