Improve sock_diag debugging code.

- Unbreak GetConstantName on modules that contain constants whose
  names start with "INET_DIAG_BC".
- Honour NL_DEBUG when sending dump requests.
- Decode sock_diag messages differently depending on whether
  they are requests or responses.
- Print sock_diag bytecode hex-encoded instead of raw.

(cherry picked from commit c5673f350e1e38b101a4cceb221a7beddc89ba78)

Change-Id: I3cc20ab635d1ff4f137aa0f9ff0ff88b1a37ce72