API: flux_msg_get_payload() should return const payload pointer

[garlick]

Since payload is the property of const flux_msg_t *, buf should also be const:

int flux_msg_get_payload (const flux_msg_t *msg, int *flags, void **buf, int *size)

Functions based on this one also need const payload arguments:

int flux_request_decode_raw (const flux_msg_t *msg, const char **topic,
                             void **data, int *len);
int flux_response_decode_raw (const flux_msg_t *msg, const char **topic,
                              void **data, int *len);
int flux_rpc_get_raw (flux_future_t *f, void **data, int *len);
int flux_mrpc_get_raw (flux_mrpc_t *mrpc, void **data, int *len);
int flux_content_load_get (flux_future_t *f, void **buf, void **buf, int *len);