Reverted the fix for gen_subnet and make use of is_ipaddrv6

Trying to change as little as possible to make IPv6 NAT work, the gen_subnet fix
is now backed out and the ':' check is replaced by a call to the is_ipaddrv6
function that ermal pointed out to me.

Signed-off-by: Werner Johansson <wj@xnk.nu>