INT 2F - DBLSPACE.BIN - "GetExtraInfo" - DETERMINE NUMBER OF DISK_UNIT STRUCTS
	AX = 4A11h
	BX = 0009h
	DL = compressed drive number (0=A:)
Return: AX = status (see also #02804)
	    0000h successful
		CL = number of DISK_UNIT structures allocated
			  (see AX=4A11h/BX=0005h)
		CH = DoubleGuard enabled-checks bitflags in bits 6-0 (DOS 6.2)
Note:	the DoubleGuard checks are enabled or disabled as a block by the
	  DoubleGuard= line in DBLSPACE.INI; they may be individually set with
	  the CheckSum= line.
SeeAlso: AX=4A11h/BX=0000h,AX=4A11h/BX=0008h