bpf: sockmap, code sockmap_test in C

By moving sockmap_test from shell script into C we can run it directly
from selftests, but we can also push the input/output around in proper
structures.

However, keep the CLI options around because they are useful for
debugging when a paticular pattern of msghdr or sockmap options
trips up the sockmap code path.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>