Ossama Othman

Leverage the MPTCP [`MPTCP_ATTR_IF_IDX`](https://github.com/multipath-tcp/mptcp/blob/mptcp_v0.95/include/uapi/linux/mptcp.h#L41) generic netlink API attribute to potentially simplify or entirely remove network interface index look-ups in the mptcpd plugin framework.

The MPTCP path manager [generic netlink API](https://github.com/multipath-tcp/mptcp/commit/4ea5dee) in the multipath-tcp.org kernel provides event filtering. Implement MPTCP event filtering support in `mptcpd`.