ip6addrctl(8): Teach ip6addrctl to attach and run itself in a jail

This will make it easier to manage address selection policies of vnet
jails, especially for those light weighted OCI containers or slim jails.

Requested by: dfr
Reviewed by: dfr, markj
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D48679