net: make WriteMsg{IP,UDP,Unix} return the correct number of bytes transferred
net: make WriteMsg{IP,UDP,Unix} return the correct number of bytes transferred

Fixes issue 7645

LGTM=iant, bradfitz
R=iant, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/90170046

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