IPV6_PKTINFO support for v4-mapped IPv6 sockets

When using v4-mapped IPv6 sockets with IPV6_PKTINFO we do not
respect the given v4-mapped src address on the IPv4 socket.
Implement the needed functionality. This allows single-socket
UDP applications (such as OpenVPN) to work better on FreeBSD.

Requested by:	Gert Doering (gert greenie.net), pfsense
Tested by:	Gert Doering (gert greenie.net)