This CL disables the use of ip_mreqn with IP_{ADD,DROP}_MEMBERSHIP on
FreeBSD due to some test flakiness on some kernel versions that support
IGMPv3. It will be fixed once we support IGMPv3 related features such
as source-specific multicasting and source filtering.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/94670043