netstat: restore printing the "default" keyword, provide -nn option

Avoid POLA breakage and preserve output standard that really predates
the FreeBSD project itself. There are scripts in the wild that rely
on the behavior.

Provide option to specify -nn twice to have a completely numeric
output of the routing tables.

Fixes: 9206c79961986c2114a9a2cfccf009ac010ad259
This reverts commit e090646d6f5a4a6848ecd4bcb1f2db498ea3b3e2.

Reviewed by: zlei, gallatin, melifaro, allanjude, markj, emaste
Differential Revision: https://reviews.freebsd.org/D48729