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