netfilter: nft_log: complete logging support

Use the unified nf_log_packet() interface that allows us explicit
logger selection through the nf_loginfo structure.

If you specify the group attribute, this means you want to receive
logging messages through nfnetlink_log. In that case, the snaplen
and qthreshold attributes allows you to tune internal aspects of
the netlink logging infrastructure.

On the other hand, if the level is specified, then the plain text
format through the kernel logging ring is used instead, which is
also used by default if neither group nor level are indicated.

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