Let the kernel delete the (default) route when we deconfigure the

Let the kernel delete the (default) route when we deconfigure the
interface.

This works around a problem where the kernel always deletes
the first default route if there are multiple present
with the same gateway.

This only fixes the problem when running ifconfig inet -autoconf.

There are other cases where we call configure_rotures(RTM_DELETE), for
example when setting ignore routes in dhcpleased.conf and issuing a
reload. To fix that we either need help from the kernel to distinguish
routes by IFP or track priorities and hope they are unique.

Problem reported by mbuhl.
OK claudio