mcproxy: fix detection of interface for IPv4 and source address for IPv6
Two similar issues fixed for related IP 4/6 functions:
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Get an explicit interface number for all packets instead of detecting interface by the source address. Some proxies may send IGMP reports with a zero IP source address (0.0.0.0), so interface number can't be reliably detected by source IP.
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Instead of the source address, the destination address from IPV6_PKTINFO was used. This incorrect address was included in MLD multicast dump entries. Now the real src address of the packet is used.