evaluate: do not allow to list/flush anonymous sets via list command

evaluate: do not allow to list/flush anonymous sets via list command

Don't allow this:

 # nft list set x __set0
 table ip x {
        set __set0 {
                type ipv4_addr
                flags constant
                elements = { 1.1.1.1 }
        }
 }

Constant sets never change and they are attached to a rule (anonymous
flag is set on), do not list their content through this command. Do not
allow flush operation either.

After this patch:

 # nft list set x __set0
 Error: No such file or directory
 list set x __set0
            ^^^^^^

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