464xlat: use a gatewayed route, not point-to-point

Various applications such as Skype and our legacy VPN code
do not understand routes pointed directly at point-to-point
interfaces and require a default gateway IPv4 address in
order to function.

Grudgingly accept that routes without default gateways Are Hard
and use gatewayed routes instead. This causes routing to go from:

default dev clat4  scope link

to:

default via 192.0.0.4 dev clat4  scope link
192.0.0.4 dev clat4  scope link

and those apps now work.

Bug: 9597256
Bug: 9597516
Change-Id: I7b7890873802d3cb99affd6eb70b8ab75e7a2cf6