INT 21,3D - Open File Using Handle

	AH = 3D
	AL = open access mode
	     00  read only
	     01  write only
	     02  read/write
	DS:DX = pointer to an ASCIIZ file name

	on return:
	AX = file handle if CF not set
	   = error code if CF set  (see DOS ERROR CODES)


	Access modes in AL:

	|7|6|5|4|3|2|1|0|  AL
	 | | | | | `-------- read/write/update access mode
	 | | | | `--------- reserved, always 0
	 | `-------------- sharing mode (see below) (DOS 3.1+)
	 `--------------- 1 = private, 0 = inheritable (DOS 3.1+)


	Sharing mode bits (DOS 3.1+):	       Access mode bits:
	654				       210
	000  compatibility mode (exclusive)    000  read access
	001  deny others read/write access     001  write access
	010  deny others write access	       010  read/write access
	011  deny others read access
	100  full access permitted to all


	- will open normal, hidden and system files
	- file pointer is placed at beginning of file