tor_addr_is_public_for_reject should return 0 for multicast addresses

This makes the treatment of addresses from other sources consistent with the implementation of get_interface_address6_list.

This might need to be cherry-picked to:

• #17027 (moved) (0.2.7)
• #17610 (moved) (0.2.6) But only once their implementations include tor_addr_is_public_for_reject.