net: return correct point-to-point interface address on linux
net: return correct point-to-point interface address on linux

On Linux point-to-point interface an IFA_ADDRESS attribute
represents a peer address. For a correct interface address
we should take an IFA_LOCAL attribute instead.

Fixes issue 4839.

R=golang-dev, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7352045

gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go