src: add support to command "destroy"

src: add support to command "destroy"

"destroy" command performs a deletion as "delete" command but does not fail
if the object does not exist. As there is no NLM_F_* flag for ignoring such
error, it needs to be ignored directly on error handling.

Example of use:

	# nft list ruleset
        table ip filter {
                chain output {
                }
        }
        # nft destroy table ip missingtable
	# echo $?
	0
        # nft list ruleset
        table ip filter {
                chain output {
                }
        }

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>