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.

MFC after: 3 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D17100