Compatibility: 6.0+
Expects: AX    4a11H (mux number)
BX    0009H (DoubleSpace fn code)
DL    any compressed drive number (0=A, 1=B, etc.)
                                                                  
Returns: AX    DoubleSpace API Error Code (0000H=success)
CL    count of DISK_UNIT structures allocated by DBLSPACE.BIN
                                                                  
Info: This determines how many compressed volumes can be mounted
simultaneously.

This is related to the value of the MaxRemovableDrives setting in
DBLSPACE.INI.  This fn returns that number plus the number of
successful ActivateDrive lines in DBLSPACE.INI.

- -

INT 2fH 4a11H BX=0009H: Get Count of DISK_UNIT Structures