This change is intended to address @glebius comments from the original D55663.
ip6_hdr_pseudo{} is referenced by certain OpenBSD OCF related components. I am
using aligned(4) and not packed as urged by the late Hans-Petter Selasky.
Use C99 types and style. We must eat the churn now cross-BSD compatibility
is "Fade to Black".
Put _Static_assert under #ifdef INVARIANTS to not disrupt regular compilation,
as this resides in a commonly included header file.