src: add comment support for objects

src: add comment support for objects

Enables specifying an optional comment when declaring named objects. The
comment is to be specified inside the object's block ({} block)

Relies on libnftnl exporting nftnl_obj_get_data and kernel space support
to store the comments.

For consistency, this patch makes the comment be printed first when
listing objects.

Adds a testcase importing all commented named objects except for secmark,
although it's supported.

Example: Adding a quota with a comment

> add table inet filter
> nft add quota inet filter q { over 1200 bytes \; comment "test_comment"\; }
> list ruleset

table inet filter {
	quota q {
		comment "test_comment"
		over 1200 bytes
	}
}

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