DEF_TAPD returns default tape drive names as a scaler string. It includes default entries for the
most common operating systems. It offers automatic substitution of the an
input drive number (default is drive 0). If the system variable !ys_deftape is defined,
the routine will return that value (with drive number substitution, if requested).
IDL> drive = def_tapd()
IDL> drive = def_tapd(2)