nftables: Add more operations and raise kernel errors

[blechschmidt]

Hello,

this PR is motivated by the lack of error reporting for nftables write operations. I have seen that there are unfinished classes like BatchSocket and BatchAddrPool, which are probably supposed to tackle the issue which I am addressing here. So I hope that this PR is not too intrusive or diverging too much from your intention.

Currently, the handling of batch messages is difficult, as nlm_request sends each message separately and listens for responses from a single message sequence number only. Therefore, 39741a4 introduces nlm_request_batch, which can be supplied with a list of messages. The function will then listen for expected responses for any of the supplied messages and return/yield them to the caller.

This is leveraged by 729de2d, which introduces acknowledgements for write operations by default. With this commit, errors, e.g. caused by invalid rule expressions or missing NLAs, will no longer go unnoticed and silent failures will no longer happen. The commit also adds some additional commands for the table, chain and rule objects.

Before, the following would silently fail because the chain does not exist:
nft.rule('add', table='test', chain='doesnotexist', expressions=some_valid_expr)

Now it will fail as indicated by the errno from the kernel:
pr2modules.netlink.exceptions.NetlinkError: (2, 'No such file or directory')

Silent failures can still be enabled by setting the flags parameter to 0. If the parameter is not set, it will default to NLM_F_ACK, which will cause the batch request function to expect an acknowledgement:
nft.rule('add', table='test', chain='doesnotexist', expressions=some_expr, flags=0)

Kind regards.

[svinota]

• formatting issues — I have to update the contribution docs, thanks for the reminder
• nlm_request_batch — there was a plan to introduce something like BatchSocket.commit() also

Let's merge your code first: It's better to have something working and used, and thereafter we can eventually employ other possible solutions, if there will be any.

Thanks for the contribution!