samples/bpf: all XDP samples should unload xdp/bpf prog on SIGTERM

It is common XDP practice to unload/deattach the XDP bpf program,
when the XDP sample program is Ctrl-C interrupted (SIGINT) or
killed (SIGTERM).

The samples/bpf programs xdp_redirect_cpu and xdp_rxq_info,
forgot to trap signal SIGTERM (which is the default signal used
by the kill command).

This was discovered by Red Hat QA, which automated scripts depend
on killing the XDP sample program after a timeout period.

Fixes: fad3917e ("samples/bpf: add cpumap sample program xdp_redirect_cpu")
Fixes: 0fca931a ("samples/bpf: program demonstrating access to xdp_rxq_info")
Reported-by: Jean-Tsung Hsiao <jhsiao@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>