Add KPI that can be used by tunneling interfaces to handle IP addresses
appearing and disappearing on the host system.

Such handling is need, because tunneling interfaces must use addresses,
that are configured on the host as ingress addresses for tunnels.
Otherwise the system can send spoofed packets with source address, that
belongs to foreign host.

The KPI uses ifaddr_event_ext event to implement addresses tracking.
Tunneling interfaces register event handlers and then they are
notified by the kernel, when an address disappears or appears.

ifaddr_event_compat() handler from if.c replaced by srcaddr_change_event()
in the ip_encap.c

Sponsored by:	Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D17134