pf: allow states to be killed by their pre-NAT address

If a connection is NAT-ed we could previously only terminate it by its
ID or the post-NAT IP address. Allow users to specify they want look for
the state by its pre-NAT address. Usage: pfctl -k nat -k <address>.

See also: https://redmine.pfsense.org/issues/11556
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42312