iOS: UDP multicast group joining incompatibilities

Submitted by Sebastian Dröge @sdroege

Link to original bug (#775873)

Description

https://lists.freedesktop.org/archives/gstreamer-devel/2016-December/061928.html

It seems like on iOS, IP_ADD_SOURCE_MEMBERSHIP and MCAST_JOIN_GROUP do slightly different things. GIO does the former, the latter actually seems to work in all cases.

To me this seems like a bug in Apple's code, but we should probably work around this by always doing the latter on iOS (and possibly macOS? It's the same kernel and system libraries after all). Any opinions on this, are you aware of any other differences between these two sockopts?

I would prepare a patch if this is considered like a sensible approach.