MFC r339537:
Add ifaddr_event_ext event. It is similar to ifaddr_event, but the handler receives the type of event IFADDR_EVENT_ADD/IFADDR_EVENT_DEL, and the pointer to ifaddr. Also ifaddr_event now is implemented using ifaddr_event_ext handler. Sponsored by: Yandex LLC Differential Revision: https://reviews.freebsd.org/D17100