make $pve_std_chains a copy of $pve_std_chains_conf

create a new $pve_std_chains with $pve_std_chains_conf as template on
every compilation of the rules. This avoids persitant changes to the
$pve_std_chains and makes it easier to read the std_chains configuration
from external config files (later to implement).

Signed-off-by: Tom Weber <pve@junkyard.4t2.com>