net: enable SO_REUSEADDR, SO_REUSEPORT options on stream, multicast listeners only

This CL changes default SOL_SOCKET settings to mitigate connect
failure on OpenBSD or similar platforms which support randomized
transport protocol port number assignment.

Fixes #2830.

R=rsc, jsing
CC=golang-dev
https://golang.org/cl/5648044
6 files changed