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

(cherry picked from commit 93abcf17e6cf3c1cd8511c8ff7a8bf20b2d76367)