netinet: get interface event notifications directly via EVENTHANDLER(9)

The old mechanism of getting them via domains/protocols control input
is a relict from the previous century, when nothing like EVENTHANDLER(9)
existed yet. Retire PRC_IFDOWN/PRC_IFUP as netinet was the only one
to use them.

Reviewed by: melifaro
Differential revision: https://reviews.freebsd.org/D36116