rtsock: subscribe to ifnet eventhandlers instead of direct calls.
Stop treating rtsock as a "special" consumer and use already-provided
ifaddr arrival/departure notifications.
MFC after: 2 weeks
Test Plan:
21:05 [0] m@devel0 route -n monitor -> ifconfig vtnet0.2 create got message of size 24 on Tue Aug 9 21:05:44 2022 RTM_IFANNOUNCE: interface arrival/departure: len 24, if# 3, what: arrival got message of size 168 on Tue Aug 9 21:05:54 2022 RTM_IFINFO: iface status change: len 168, if# 3, link: up, flags:<BROADCAST,RUNNING,SIMPLEX,MULTICAST> -> ifconfig vtnet0.2 destroy got message of size 24 on Tue Aug 9 21:05:54 2022 RTM_IFANNOUNCE: interface arrival/departure: len 24, if# 3, what: departure
Reviewed By: glebius
Differential Revision: https://reviews.freebsd.org/D36095
MFC after: 2 weeks
(cherry picked from commit d8b42ddcac5cb86af679968d09c45c9a7cc3e4fb)