pf: Support killing 'matching' states

pf: Support killing 'matching' states

Optionally also kill states that match (i.e. are the NATed state or
opposite direction state entry for) the state we're killing.

See also https://redmine.pfsense.org/issues/8555

Submitted by:	Steven Brown
Reviewed by:	bcr (man page)
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/D30092