Use the appropriate call to getsockopt for IPv6 sockets

The original call to getsockopt to know the original address on transparently
proxyed sockets using REDIRECT in iptables failed with IPv6 addresses because
it assumed all sockets used IPv4.

This patch fixes this by using the appropriate options and adding the headers
containing the needed definitions for these.

This patch is released under the same license as the original file as
long as the author iscredited.

Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>