py: add Nftables.{get,set}_input_flags() API

py: add Nftables.{get,set}_input_flags() API

Similar to the existing Nftables.{get,set}_debug() API.

Only notable (internal) difference is that nft_ctx_input_set_flags()
returns the old value already, so we don't need to call
Nftables.get_input_flags() first.

The benefit of this API, is that it follows the existing API for debug
flags. Also, when future flags are added it requires few changes to the
python code.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>