samples/bpf: add -c/--copy -z/--zero-copy flags to xdpsock

The -c/--copy -z/--zero-copy flags enforces either copy or zero-copy
mode.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>