This CL revises existing platform-dependent default socket
options to make it possible to accomodate multiple multicast
datagram listeners on a single service port.

Also removes unnecessary SO_REUSEADDR, SO_REUSEPORT socket
options from unicast datagram sockets by default.

Fixes #1692.

R=devon.odell, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5538052