Add a new function is_default_interface() which determines if this
interface is one with the default route (or there isn't one). Use it to
decide if we should adjust the default route and /etc/resolv.conf.

Fix the delete of the default route. The if statement was totally bogus
and the delete only worked due to a typo. [1]

Reported by: Jordan Coleman <jordan at JordanColeman dot com> [1]
MFC after: 1 week