Draft: [th/platform-genl-3] add async generic netlink socket and use it for genl family ID

netlink sockets can send notifications. This also applies to generic netlink families. For MPTCP, we will want to receive notification, and to handle that, we will need to watch the file descriptor and handle async genl events -- pretty similar like we do for rtnetlink already.

Add code for doing that.

As proof of concept (and because it seems useful), use this to track the generic family IDs (instead of fetching them when needed).