Attach programs to tcx

[qmonnet]

For networking programs, we support attaching to XDP (bpftool net attach) but not to tc. This is because attaching to tc would require supporting the creating of tc qdiscs and filters, and eventually recreated a good chunk of tc from iproute2, which is not exactly suitable.

However, we now have tcx programs that should be simpler to attach, and for which we could maybe add support in bpftool (to be confirmed).