tests: add table 'use' attr to testfiles

Parsing tests were failing because a missing 'use' attribute in tables.

validating xmlfiles/02-table.xml: FAILED
from file:     0</flags></table>
f
from snprintf: 0</flags><use>0</use
                         ^
[...]

validating jsonfiles/64-ruleset.json: FAILED
from file:     ,"flags":0}},{"chain
from snprintf: ,"flags":0,"use":0}}
                         ^

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
examples: nft-set-get: retrieve all sets via unspec

Other nftables objects are allowed to be dumped with NFPROTO_UNSPEC.
With sets is also possible since kernel patch c9c8e48 ("netfilter: nf_tables:
dump sets in all existing families").

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
examples: add nft-ruleset-get

This example prints the ruleset, using the ruleset API of nftables.

The kernel patch c9c8e48 ("netfilter: nf_tables: dump sets in all existing
families") is required.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>