MFC sys/netinet6: Implement RFC 7217 (private stable addresses)

[madpilot]

Summary

I'd like to MFC my RFC 7217 implementation to 15-stable.

As suggested by @zarychtam_plan-b.pwste.edu.pl I'm also including SLAAC patches ([1] and followup fix [2]) from @linnemannr_gmail.com, to allow a clean merge without conflicts.

I think this is the cleanest choice, otherwise I could merge my patches only fixing conflicts, but then there would be further conflicts if [1] and [2] are to be merged later.

I'm including everyone who approved the previous patches as reviewers, plus @linnemannr_gmail.com and @kp as author and commiter of the extra patches I'm planning to bring along.

ALso, I plan to merge each commit as a single cherry pick and push them in one go. Looks to me like the best way, if merging them in a single commit is preferred please let me know.

Test Plan

I've tested the patch in bhyve VMs and everything looks fine.

I'm also leveraging this code daily on head.

[madpilot]

Forgot to add references:

[1] 9e792f7ef7298080c058fbc2d36a4e60e596dae9
[2] f9fc93690aef7a56f62a051de6231fe2af699728