pfctl: change for af-to / NAT64 support.

The general syntax is:
pass in inet from any to 192.168.1.1 af-to inet6 from 2001::1 to 2001::2
In the NAT64 case the "to" is not needed in af-to and the IP is extraced
from the IPv6 dst (assuming a /64 prefix).
Again most work by sperreault@, mikeb@ and reyk@
OK mcbride@, put it in deraadt@

Obtained from: OpenBSD, claudio <claudio@openbsd.org>, 0cde32ce3f
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47790