Support struct ip_mreqn as argument for IP_ADD_MEMBERSHIP. Legacy support for struct ip_mreq remains in place. The struct ip_mreqn is Linux extension to classic BSD multicast API. It has extra field allowing to specify the interface index explicitly. In Linux it used as argument for IP_MULTICAST_IF and IP_ADD_MEMBERSHIP. FreeBSD kernel also declares this structure and supports it as argument to IP_MULTICAST_IF since r170613. So, we have structure declared but not fully supported, this confused third party application configure scripts. Code handling IP_ADD_MEMBERSHIP was mixed together with code for IP_ADD_SOURCE_MEMBERSHIP. Bringing legacy and new structure support into the mess would made the "argument switcharoo" intolerable, so code was separated into its own switch case clause.