bpf: add TCP_SAVE_SYN/TCP_SAVED_SYN sample program

Sample program which shows TCP_SAVE_SYN/TCP_SAVED_SYN usage example:
bpf program which is doing TOS/TCLASS reflection (server would reply
with a same TOS/TCLASS as client).

Signed-off-by: Nikita V. Shirokov <tehnerd@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>