src: add input flags for nft_ctx

src: add input flags for nft_ctx

Similar to the existing output flags, add input flags. No flags are yet
implemented, that will follow.

One difference to nft_ctx_output_set_flags(), is that the setter for
input flags returns the previously set flags.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Reviewed-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>