Add xdpsock code to xdp-tools

[tohojo]

This WiP PR imports the code from the xdpsock utility in the bpf_examples repository and incorporates the functionality into xdp-bench (drop and l2fwd) and xdp-trafficgen (tx).

The xdpsock code is transformed into library functions that can be called from both the xdp-bench and xdp-trafficgen binaries, and the options are converted into the xdp-tools parameter framework.