libknot/xdp: extend filter configuration with DROP mode

src/libknot/xdp/af_xdp.c

src/libknot/xdp/af_xdp.h

src/libknot/xdp/bpf-kernel-obj.c

src/libknot/xdp/bpf-kernel.c

src/utils/xdp-gun/main.c