This change retires two historic relics: the if_afdata[] array and the
dom_ifattach/dom_ifdetach methods.
The if_afdata[] array is a relic of the era, when there was expectation
that many transport protocols will coexist with IP, e.g. IPX or NetAtalk.
The array hasn't had any members except AF_INET and AF_INET6 for over a
decade already. This change removes the array and just leaves two pointer
fields: if_inet and if_inet6.
The dom_ifattach/dom_ifdetach predates the EVENTHANDLER(9) framework and
was a good enough method to initialize protocol contexts back then. Today
there is no good reason to treat IPv4 and IPv6 stacks differently to other
protocols/features that attach and detach from an interface.
The locking of if_afdata[] is a relic of SMPng times, when the system
startup and the interface attach was even more convoluted than before this
change, and we also had unloadable protocols that used a field in
if_afdata[]. Note that IPv4 and IPv6 are not unloadable.
Note that this change removes NET_EPOCH_WAIT() from the interface detach
sequence. This may surface several new races associated with interface
removal. I failed to hit any with consecutive test suite runs, though.
The expected general race scenario is that while struct ifnet is freed
with proper epoch_call(9) itself, some structures hanging off ifnet are
freed with direct free(9). The proper fix is either make if_foo point at
some static "dead" structure providing SMP visibility of this store, or
free those structure with epoch_call(9). All of these cases are planned
to be found and resolved during 16.0-CURRENT lifetime.