iptables: fixes for /sbin symlinks

We may need to overwrite them in some situations in postinst.
Also, deconfigure them in more cases in prerm. 

Closes: #913811
Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>

debian/iptables.postinst

@@ -13,7 +13,7 @@ if [ "$1" = "configure" ] ; then
 
 	for i in $LIST ; do
 		if [ ! -e "$i" ] ; then
-			ln -s /usr$i $i
+			ln -sf /usr$i $i
 		fi
 	done
 fi