net: make Dial and Listen behavior consistent across over platforms
This CL changes the behavior of Dial and Listen API family.
Previous Dial and Listen allow a combo of "tcp6" and IPv4 or IPv6
IPv4-mapped address as its argument, but it also makes slightly
different behaviors between Linux and other platforms. This CL fixes
such differences across over platforms by tweaking IP-level socket
option IPV6_V6ONLY. Consequently new Dial and Listen API family will
reject arguments consists of "tcp6" and IPv4 or IPv6 IPv4-mapped
address.
This CL also adds a bit clarified unicast listener tests.
Fixes #2581.
R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/
5677086