spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

pfsync: prepare code to accommodate AF_INET6 family

Work is ongoing to add support for pfsync over IPv6. This required some
changes to allow for differentiating between the two families in a more
generic way.

This patch converts the relevant ioctls to using nvlists, making future
extensions (such as supporting IPv6 addresses) easier.

Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D36277