INT 21 - DOS 1+ - SELECT DEFAULT DRIVE

	AH = 0Eh
	DL = new default drive (00h = A:, 01h = B:, etc)
Return: AL = number of potentially valid drive letters
Notes:	under Novell NetWare, the return value is always 32, the number of
	  drives that NetWare supports
	under DOS 3.0+, the return value is the greatest of 5, the value of
	  LASTDRIVE= in CONFIG.SYS, and the number of drives actually present
	on a DOS 1.x/2.x single-floppy system, AL returns 2 since the floppy
	  may be accessed as either A: or B:
	otherwise, the return value is the highest drive actually present
	DOS 1.x supports a maximum of 16 drives, 2.x a maximum of 63 drives,
	  and 3+ a maximum of 26 drives
	under Novell DOS 7, this function returns the correct LASTDRIVE value
	  even when the undocumented LASTDRIVE=27..32 directive was used in
	  CONFIG.SYS
	"parse FCB" (see AH=29h) can be used to determine whether a drive
	  letter is valid
SeeAlso: AH=19h,AH=3Bh,AH=DBh