No logner set an IPv4 loopback address by default in defaults/rc.conf.
If not specified, network.subr will add it automatically if we have
INET support (1).

In network.subr only call the address family up/down functions
if the respective AF is available.

Switch to new kern.features variables for inet and inet6 as the
inet sysctl tree is also available for IPv6-only kernels leading
to unexpected results.

Suggested by: hrs (1)
Reviewed by: hrs
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
MFC after: 20 days