MFC r349927, r349929:

MFC r349927, r349929:

r349927:
  Resolve IPv6 checksum errors with stateful inspection. According to
  PR/203585 this appears to have been broken by r235959, which predates
  the ipfilter 5.1.2 import into FreeBSD.

  The IPv6 checksum calculation is incorrect. To resolve this we call
  in6_cksum() to do the the heavy lifting for us, through a new function
  ipf_pcksum6(). Should we need to revisit this area again, a DTrace probe
  is added to aid with future debugging.

  Plus whitespace adjustments (r348989).

  PR:		203275, 203585
  Differential Revision:	https://reviews.freebsd.org/D20583

r349929:
  Move the new ipf_pcksum6() function from ip_fil_freebsd.c to fil.c.
  The reason for this is that ipftest(8), which still works on FreeBSD-11,
  fails to link to it, breaking stable/11 builds.

  ipftest(8) was broken (segfault) sometime during the FreeBSD-12 cycle.
  glebius@ suggested we disable building it until I can get around to
  fixing it. Hence this was not caught in -current.

  The intention is to fix ipftest(8) as it is used by the netbsd-tests
  (imported by ngie@ many moons ago) for regression testing.

git-svn-id: svn://svn.freebsd.org/base/stable/10@349931 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f