INT 21 - DOS 4.0+ - GET BOOT DRIVE

	AX = 3305h
Return: DL = boot drive (1=A:,...)
Notes:	This function does not use any of the DOS-internal stacks and may
	   thus be called at any time.  It is directly dispatched from
	   the INT 21h entry point with interrupts disabled.
	NEC 9800-series PCs always call the boot drive A: and assign the other
	  drive letters sequentially to the other drives in the system
	this call is supported by OS/2 Warp 3.0, but not earlier versions of
	  OS/2; it is also supported by Novell DOS 7