l2tp: Fix get_tunnel/get_session to return only requested object

[kleinweby]

L2TP does not support using filters on NLM_F_DUMP and always returns all objects. The current get_tunnel/session functions however always use NLM_F_DUMP and set the tunnel/session ids if provided by the caller. These are simply ignored.

This patch fixes get_tunnel and get_session to always require specifying the desired tunnel/session id to always get one requested object. If the requested object does not exist an NetlinkError is thrown.

To also provide a way to get all tunnels/sessions dump_tunnels and dump_sessions has been introduced.

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Note that does does change the signature of get_tunnel and get_session. I've also adjusted the return value to return the one l2tpmsg message we get, so users don't need a odd [0] anymore.

[svinota]

I'm working on fixing the CI pipeline, the PR will be merged this weekend

[doeringudh]

I believe there might have happened an oversight with regard to the parameters for the new function dump_tunnels() in pyroute2/netlink/generic/l2tp.py when merging this PR's original patch. Said function has a parameter "tunnel_id" that is actually never used, and also would not make sense in this context.

Now compare it with the also new and related dump_sessions(), which correctly does not have any parameters, apart from "self". In my view dump_tunnels() should have the same signature.