* setsockopt(s, IPV6_ONLY, &one, sizeof(int)) will disable IPV6-only
mode. Incoming/outgoing IPV4 packets are dropped.
* Otherwise, sockets bound to IPV6_ANY_ADDR can receive unicast packets
for IPV4 or IPV6.
* sendto() a IPV6-mapped-IPV4 address on a UDP socket works correctly
(not supported for RAW or TCP sockets.)
* getaddrinfo() option AI_V4MAPPED is implemented.
As well as extending support to TCP & RAW, there is some potential improvement
to dropping incoming packets - the drop happens a bit late in the process and
there is no "ICMP port unreachable" response sent.
