Backport: Use iptables to list rules

Since Debian Buster, old iptables/xtables (legacy) and new nftables frameworks can be used in parallel.
We chose to use the legacy framework, but since the introduction of Docker in the SDK, which set its specific rules using nftables, this breaks the connman tethering as there are two sets running.

The iptables tools are now use in place of iptables-legacy tools to use the nftables only.

Also removes deprecated lxcbr0 rules check.

https://phabricator.apertis.org/T8420

Signed-off-by: Frédéric Danis frederic.danis@collabora.com