--------D-2169-------------------------------
INT 21 U - DOS 4.0+ internal - GET/SET DISK SERIAL NUMBER
	AH = 69h
	AL = subfunction
	    00h get serial number
	    01h set serial number
	BL = drive (0=default, 1=A, 2=B, etc)
	BH = info level (00h only for DOS; OS/2 allows other levels)
	DS:DX -> disk info (see #01766)
Return: CF set on error
	    AX = error code (see #01680 at AH=59h/BX=0000h)
	CF clear if successful
	    AX destroyed
	    (AL = 00h) buffer filled with appropriate values from extended BPB
	    (AL = 01h) extended BPB on disk set to values from buffer

The serial number is computed from the current date and time when the disk is
created; the first part is the sum of the seconds/hundredths and month/day,
the second part is the sum of the hours/minutes and year.
