avformat/udp: always use IP_ADD_SOURCE_MEMBERSHIP for subscribing to an UDP multicast source group in IPv4

That alone supports specifying the interface based on its address. Getting the
interface index from the local address seems quite a bit of work in a platform
independent way...

Obviously for IPv6 we still always use MCAST_JOIN_SOURCE_GROUP.

As a side effect this also fixes ticket #7459.
Signed-off-by: Marton Balint <cus@passwd.hu>