Retire most of the classful network behaviour of netstat -r output, for IPv4.

Without -n, we now only print a "network name" without the prefix length
under the following conditions:

1. the network address and mask matches a classful network prefix;
2. getnetbyaddr(3) returns a network name for this network address.
   
   With -n, we unconditionally print the full unabbreviated CIDR network prefix in the form "a.b.c.d/p". 0.0.0.0/0 is still printed as "default".

This change is in preparation for changes such as equal-cost multipath, and
to more generally assist operational deployment of FreeBSD as a modern IPv4
router. There are currently no plans to backport this change.

Discussed on: freebsd-net