net: fix TestFirstFavoriteAddr failure on single IP stack kernel
net: fix TestFirstFavoriteAddr failure on single IP stack kernel

Update issue 3610
Update issue 5267
Update issue 5707

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/13465043

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang