Always create loopback routes on every fib, for both IPv4 and IPv6

etc/rc.d/routing
Create loopback IPv4 and IPv6 routes on every fib at boot. Revert
278302; now that all FIBs have IPv6 loopback routes, the "route add
-reject" commands won't fail.

tests/etc/rc.d/routing_test.sh
Greatly simplify static_ipv6_loopback_route_for_each_fib. It was
written under the assumption that loopback routes would be added to
a given fib by the kernel as soon as an interface is configured on
that fib. However, the logic can be much simpler now that we simply
add loopback routes to all fibs at boot. This also removes the need
to run the test as root, removes the restriction that
net.add_addr_allfibs=0, and removes the need to configure fibs in
kyua.conf.

Also, add a test case for IPv4 loopback routes