Add rib subscription API.

Currently there is no easy way of subscribing for the routing table changes.
The only existing way is to set ifa_rtrequest callback in the each protocol
ifaddr, which is not convenient or extandable.

This change provides generic notification subscription mechanism, that will
replace current ifa_rtrequest one and allow other applications such as
accelerated routing lookup modules subscribe for the changes.

In particular, this change provides 2 hooks: 1) synchronous one
(RIB_NOTIFY_IMMEDIATE), called under RIB_WLOCK, which ensures exact
ordering of the changes and 2) async one, (RIB_NOTIFY_DELAYED)
that is called after the change w/o holding locks. The latter one does not
provide any notification ordering guarantee.

Differential Revision: https://reviews.freebsd.org/D25070