INT 21 - DOS 1+ - OPEN FILE USING FCB

	AH = 0Fh
	DS:DX -> unopened File Control Block (see #01345,#01346)
Return: AL = status
	    00h successful
	    FFh file not found or access denied
Notes:	(DOS 3.1+) file opened for read/write in compatibility mode
	an unopened FCB has the drive, filename, and extension fields filled
	  in and all other bytes cleared
	not supported by MS Windows 3.0 DOSX.EXE DOS extender
	DR DOS checks password attached with AX=4303h
	(FAT32 drive) this function will only succeed for creating a volume
	  label; FAT32 does not support FCBs for file I/O
BUG:	APPEND for DOS 3.3+ corrupts DX if the file is not found
SeeAlso: AH=10h,AH=16h,AH=3Dh,AX=4303h