src: convert set to list expression

src: convert set to list expression

The following definition:

 define xyz = { "dummy0", "dummy1" }

is represented as a set expression to ease integration with sets.

However, this definition can be used in chains and flowtables to specify
the devices, for instance:

  table netdev x {
    chain y {
      type filter hook ingress devices = $xyz priority 0; policy drop;
    }
  }

in this context, $xyz defines a _list_ of devices, not a set.

Transform the set to list expression from the evaluation step for chains
and flowtables.

This patch also handles:

 define xyz = { "dummy0", $abc }

where $abc is also transformed to a list expression in the context of
chains and flowtables.

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