IPv6 APIs rework

[Robin Jarry]

Stephen Hemminger, Jul 20, 2024 at 22:26:
> There is no need for packing or alignment in in6_addr or current DPDK, 
> what would be the benefit?  Compilers generate worse code if 
> a structure is marked packed.

The only benefit is to maintain current behaviour.

At first, I had not packed nor aligned anything and I had tons of test 
errors because the compiler added padding in structures that contained 
IPv6 addresses.

I don't want to mix things together. In my opinion, removing that 
alignof(1) constraint is an optimization which has nothing to do with 
the IPv6 API functional rework.

So my proposal is: add a structure *packed and unaligned* first so that 
*all tests are passing*.

And *then*, after the changes have been applied on the main branch and 
no critical issues have been reported, see if we need to remove these 
packed and unaligned constraints.