ddb: rename show arptab to show routes

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

show arptab command of ddb is now inappropriate
because it actually dumps routes but arp entries
aren't routes anymore. Though, the feature is useful.
So I'd like to remain it as renamed to show routes
and move the code from if_arp.c to route.c.

How about the change? Comments?

Here is a patch:
  http://www.netbsd.org/~ozaki-r/ddb-show-routes.diff

Thanks,
  ozaki-r