src: add comment support when adding tables

src: add comment support when adding tables

Adds userdata building logic if a comment is specified when creating a
new table. Adds netlink userdata parsing callback function.

Relies on kernel supporting userdata for nft_table.

Example:

> nft add table ip x { comment "test"\; }
> nft list ruleset

table ip x {
	comment "test"
}

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>