INT 13 - FLOPPY DISK - DETECT DISK CHANGE (XT 1986/1/10 or later,XT286,AT,PS)

	AH = 16h
	DL = drive number (00h-7Fh)
	SI = 0000h (to avoid crash on AT&T 6300)
Return: CF clear if change line inactive
	    AH = 00h (disk not changed)
	CF set if change line active
	    AH = status
		01h invalid command (SyQuest)
		06h change line active or not supported
		80h drive not ready or not present
Notes:	call AH=15h first to determine whether the drive supports a change
	  line
	this call also clears the media-change status, so that a disk change
	  is only reported once
BUGS:	some versions of Award 386 Modular BIOS and AMI BIOS fail to clear
	  the media-change status
	AT&T 6300 WGS systems crash if SI <> 0 on entry.
	some pre 1986/08/04 Compaq ROM BIOS have a serious bug where this
	  function may re-configure a hard disk depending on what is located
	  at ES:[BX] and data indexed to by it. MS-DOS/PC DOS IO.SYS/IBMBIO.COM
	  install a special filter when they detect Compaq ROM BIOSes with
	  earlier dates.
	some Compaq 286 systems have a bug in all INT 13h functions >= 16h,
	  which causes the byte at DS:0074h to be destroyed when called for
	  hard disks (DL >= 80h). MS-DOS/PC DOS IO.SYS/IBMBIO.COM performs a
	  test on this bug using this sub-function, and if found installs a
	  special filter which points DS into ROM, so that it cannot cause
	  any harm.
	some drives (or controllers???) forget the change line status
	  if another drive is accessed afterwards. The DOS BIOS takes care
	  of this by not relying on the reported change line status when
	  the change line is not active and a different drive is accessed,
	  instead it reports "don't know" to the DOS kernel.
SeeAlso: AH=15h,AH=49h