Fix up default route selection by
a) Forcing user to explicitly chose 'dhcp' as a mechanism for
specifying a default route, rather than guessing based on one or more
interfaces being configured by dhcp.
b) If the user specified default route does not work, re-present the
existing default route rather than losing it.
c) Move default route selection to after nameserver activation so the
user can specify a hostname as the default route.
Change /etc/netstart so that /etc/mygate wins if a default route was
already specified (i.e. by dhcp).
ok deraadt@.