INT 21 - DOS 2+ - "DUP2", "FORCEDUP" - FORCE DUPLICATE FILE HANDLE

	AH = 46h
	BX = file handle
	CX = file handle to become duplicate of first handle
Return: CF clear if successful
	CF set on error
	    AX = error code (04h,06h) (see #01680 at AH=59h/BX=0000h)
Notes:	closes file with handle CX if it is still open
	DOS 3.30 hangs if BX=CX on entry
	moving file pointer for either handle will also move it for the other,
	  because both will refer to the same system file table
SeeAlso: AH=3Dh,AH=45h