configure: fix wrong reference to the conntrack-tools

Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
configure: rename --disable-xtables to --disable-nftables

The compat utility name is xtables, the name used for the
compatibility binary. Better explicitly refer to the compatibility
later as nftables-compat.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
configure: conditional dependencies for nftables-compat

Depend on libmnl, libnftnl, flex and bison if only if the nftables
compatibility is enabled.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
xtables-restore: remove dependency with libip4tc

Add a new operation structure, we don't actually need the libip4tc
definition.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
xtables: add xtables-compat-multi for the nftables compatibility layer

This patch should allow distributors to switch to the iptables over
nftables compatibility layer in a transparent way by updating
symbolic links from:

lrwxrwxrwx  1 root    root         13 feb  4 15:35 iptables -> xtables-multi

to:

lrwxrwxrwx  1 root    root         13 feb  4 15:35 iptables -> xtables-compat-multi

Same thing with iptables-save, iptables-restore, ip6tables, ip6tables-save,
ip6tables-restore and arptables.

Note that, after this patch, the following new symlinks are installed:

* iptables-compat
* iptables-compat-save
* iptables-compat-restore
* ip6tables-compat
* ip6tables-compat-save
* ip6tables-compat-restore
* arptables-compat

which point to the new binary xtables-compat-multi.

The idea is to keep both native and compatibility tools installed in the
system, which should also make it easier for testing purposes.

The iptables over nftables compatibility layer is enabled by default
and it requires the libmnl and libnftnl libraries. If you don't want to
compile the compatibility layer, you can still disable it through
--disable-nftables.

This patch also includes changes to adapt the existing code to this
approach.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>