The RFC specifies that under IPv6 the complete AH header must be 64 bit
aligned, and under IPv4, 32 bit aligned. Prior to this change, we (along
with other BSDs and MacOS) had violated this requirement.
This makes it possible to set up IPv6-AH between Linux and BSD, and also
probably between Windows and BSD.
Pulled directly from Maxime Villard's xform_ah.c 1.105 change, aka
b939fe2483972eb43d71bf990cfb7f26dece7839 on NetBSD/src GH.