src: add nft_ctx_output_{get,set}_stateless() to nft_ctx_output_{get,flags}_flags

src: add nft_ctx_output_{get,set}_stateless() to nft_ctx_output_{get,flags}_flags

Add NFT_CTX_OUTPUT_STATELESS flag and enable stateless printing from new
output flags interface.

This patch adds nft_output_save_flags() and nft_output_restore_flags()
to temporarily disable stateful printing

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