net: defend against broken getaddrinfo on Linux
net: defend against broken getaddrinfo on Linux

getaddrinfo is supposed to set errno when it returns
EAI_SYSTEM, but sometimes it does not.

Fixes issue 6232.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13532045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang