Use netlink multicast subscription for notification of Wireguard peer changes

[stv0g]

There has been a patch on the Wireguard mailing list add support for netlink multicast subscriptions which allow for listening to changes of the peers of an interface:

• https://lore.kernel.org/patchwork/patch/1366219/

However this patch hasnt made it up into the Linux tree yet.

We could use this for accelerating the detection of new peers or peer handshakes.

There is also a thread on the Wireguard mailing list discussing this feature:

https://lists.zx2c4.com/pipermail/wireguard/2021-March/006463.html

[stv0g]

Closing for now as the required kernel feature does not exist (yet).