INT 21 - DOS 2+ - "OPEN" - OPEN EXISTING FILE

	AH = 3Dh
	AL = access and sharing modes (see #01402)
	DS:DX -> ASCIZ filename
	CL = attribute mask of files to look for (server call only)
Return: CF clear if successful
	    AX = file handle
	CF set on error
	    AX = error code (01h,02h,03h,04h,05h,0Ch,56h) (see #01680 at AH=59h)
Notes:	file pointer is set to start of file
	if SHARE or a network is loaded, the file open may fail if the file
	  is already open, depending on the combination of sharing modes
	  (see #01403,#01404)
	file handles which are inherited from a parent also inherit sharing
	  and access restrictions
	files may be opened even if given the hidden or system attributes
	under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
	DR DOS checks the system password or explicitly supplied password at
	  the end of the filename (following a semicolon) against the reserved
	  field in the directory entry before allowing access
	sharing modes are only effective on local drives if SHARE is loaded
BUG:	Novell DOS 7 SHARE v1.00 would refuse file access in the cases in
	  #01403 marked with [1] (read-only open of a read-only file
	  which had previously been opened in compatibility mode); this was
	  fixed in SHARE v1.01 of 09/29/94
SeeAlso: AH=0Fh,AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1116h,INT 2F/AX=1226h