pfctl: allow an implicit address family for af-to rules

If the address family can be determined by the "from" or "to" parameter in the
matching part, it is no longer necessary to specify "inet" or "inet6" there.
OK henning@ mikeb@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, ff33038bc1
Sponsored by: Rubicon Communications, LLC ("Netgate")