- replace "s_flags" field by explicit boolean fields.

- "s_msg_peek" now is simplified. Previously, we would default
  to peek, unless the user caller nl_socket_disable_msg_peek()
  or set nl_socket_set_msg_buf_size(). Simplify that. We now
  default to peek, unless NL_SOCKET_FLAGS_DISABLE_MSG_PEEK is set.

  We have no callers that call nl_socket_set_msg_buf_size(),
  so we can simplify that logic and just enable peeking by default.

- keep "s_auto_ack" field, although it is always TRUE and there
  is no API to toggle that. However, it is kept as a self-documenting
  thing, so we would know the relevant places where auto-ack matters.

- drop nl_socket_disable_msg_peek(). We have no caller of this function
  and we can set peeking in nl_socket_new(). We also don't need to
  change it after creation of the socket.