inpcb: declare struct in_conninfo as a single declaration

[glebius]

Summary

This removes just one level of #define mess that is needed to reach into
an inpcbs IPv4 address. And makes the declaration easier to read. No
functional change.

[glebius]

Note: union in_dependaddr can be anonymized after small cleanup of lbgroup code and siftr(4). This removes another level of of defines for both IPv6 and IPv4. This may need universe/exp-run, though.

The idea is that instead of union in_dependaddr ie_dependfaddr, ie_dependladdr; two anonymous unions are used. Identical in their layout, but one has members with f prefix and other with l.

If there is some consensus on this being a good idea, I can run test builds and post the review,

[glebius]

The struct in_endpoints can also be anonymized easily, but that would highly likely affect something outside of FreeBSD. Another level of #define to be removed. Let me know what you think about this and I will dig deeper into this if we all agree.

Alternatively we can leave publicly declared struct in_conninfo as is and just stop using it internally. But this review (D58273) probably should go in. It must be safe.