src: update meta expr

This patch adds userspace support for the meta expression in the set flavour.

This expression indicates that the packet has to be set with a property,
currently one of mark, priority or nftrace.

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