rule: add flag to display rule handle as comment

rule: add flag to display rule handle as comment

Knowing the rule handle is necessary to be able to delete a single
rule. It was not displayed till now in the output and it was thus
impossible to remove a single rule.
This patch modify the listing output to add a comment containing
the handle when the -a/--handle flag is provided.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>