bpf: refactor sockmap sample program update for arg parsing

sockmap sample program takes arguments from cmd line but it reads them
in using offsets into the array. Because we want to add more arguments
in the future lets do proper argument handling.

Also refactor code to pull apart sock init and ping/pong test. This
allows us to add new tests in the future.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>