pf: Allow states to by killed per 'gateway'

This allows us to kill states created from a rule with route-to/reply-to
set. This is particularly useful in multi-wan setups, where one of the
WAN links goes down.

Submitted by: Steven Brown
Obtained from: https://github.com/pfsense/FreeBSD-src/pull/11/
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30058