Support capturing via fentry / fexit trace points

Linux now supports attaching BPF programs just before (fentry) and after (fexit) an XDP program.
This allows packets to be captured before and after an XDP program runs, from unmodified XDP programs (no "hook" map).

`xdpdump` already supports this, but without filtering: https://github.com/xdp-project/xdp-tools/tree/master/xdp-dump/.