src: add support for rule human-readable comments

src: add support for rule human-readable comments

This patch adds support for human-readable comments:

  nft add rule filter input accept comment \"accept all traffic\"

Note that comments *always* come at the end of the rule. This uses
the new data area that allows you to attach information to the rule
via netlink.

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