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:		[[ https://bugs.freebsd.org/203275 | 203275 ]], 203585
Differential Revision:	https://reviews.freebsd.org/D20583

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.