usr.bin/netstat: -n should not print symbolic names

In numeric mode, the default route is printed as "default" rather
than 0.0.0.0/0 or ::/0

From the man page:
"-n: Show network addresses and ports as numbers.
Normally netstat attempts to resolve addresses and ports, and display
them symbolically. If the -n option is specified, the address is
printed numerically, according to the address family.
For more information regarding the Internet IPv4 ``dot format'', refer
to inet(3). Unspecified, or `wildcard'', addresses and ports appear
as `*''."

Reported By: rgrimes
Reviewed by: emaste, ngie, eadler, seanc
Relnotes: yes
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D10320