iptables: fix save/restore symlinks with libnftnl PACKAGECONFIG enabled

When the libnftnl PACKAGECONFIG is enabled, the "iptables" symlink is correctly
points to xtables-nft-multi, however the "iptables-save" and
"iptables-restore" are still point to the xtables-legacy-multi.

So, when the "iptables" command is used it's using the nftables backend
where is the "iptables-save/restore" are using the legacy backend.

This is not consistent with other distros (e.g. Ubuntu).

The issue was identified when testing the UFW firewall with nftables backend.

Signed-off-by: Kirill Yatsenko <kiriyatsenko@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6579e433)
Signed-off-by: Steve Sakoman <steve@sakoman.com>