netinet6: factor interface addition code to the dedicated function

Summary:
Move SIOCAIFADDR_IN6 (current "primary" ioctl to add an IPv6
interface address) handling code to the dedicated in6_addifaddr()
function and make it a part of KPI. This allows in-kernel users to
add/delete interfaces addresses without relying on ioctl interface.

Subscribers: imp, ae, glebius

Differential Revision: https://reviews.freebsd.org/D36713