Attempting to join an IPv4 multicast group after binding to QHostAddress::Any fails

(note: this might also affect 4.8, I haven't checked)

See the attached diff for a test. It fails, because QHostAddress::Any is treated as a v6 bind (since it's for both v4 and v6 interfaces), but the QHostAddress passed to joinMulticastGroup is a v4 address, so it tries to take the v4 path for setsocketopt, which then fails with EINVAL (at least on OS X).

If I force it to take the v6 path, it succeeds, so I suspect we may need some additional logic to handle QHostAddress::Any binds by storing the address family, or something, but I need advice before I try handle this.