{   This procedure returns a count of tape blocks which have been passed to
{ tape block management (by calling bap$tape_bm_write_next_block) but have not
{ yet been written to the tape.
{
{       BAP$TAPE_BM_UNWRITTEN_BLK_COUNT (FILE_IDENTIFIER, COUNT, STATUS)
{
{  FILE_IDENTIFIER (Input):  The file identifier for the file.
{
{  COUNT (Output):  The number of unwritten tape blocks.
{
{  STATUS: (Output) Request status.
