evaluate: fix "list set" unexpected behaviour

evaluate: fix "list set" unexpected behaviour

Special sets like maps and flow tables have their own commands to be
listed and inspected.

Before this patch, "nft list set" was able to display these special sets
content:

  # nft list set filter test
  table ip filter {
          map test {
                  type ipv4_addr : inet_service
                  elements = { 192.168.1.101 : http-alt}
          }
  }

Now an error is shown:

  # nft list set filter test
  <cmdline>:1:1-20: Error: Could not process rule: Set 'test' does not exist
  list set filter test
  ^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Pablo M. Bermudo Garay <pablombg@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>