INT 21 - DOS 2+ - "OPEN" - OPEN EXISTING FILE
        AH = 3Dh
        AL = access and sharing modes
            bits 2-0: access mode
                000 read only
                001 write only
                010 read/write
            bit 3: reserved (0)
            bits 6-4: sharing mode (DOS 3+)
                000 compatibility mode
                001 "DENYALL" prohibit both read and write access by others
                010 "DENYWRITE" prohibit write access by others
                011 "DENYREAD" prohibit read access by others
                100 "DENYNONE" allow full access by others
            bit 7: inheritance
                if set, file is private to current process and will not be
                  inherited by child processes
        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) (see AH=59h)
Notes:  file pointer is set to start of file
        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
SeeAlso: AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1226h

File sharing behavior:
          |     Second and subsequent Opens
 First    |Compat  Deny   Deny   Deny   Deny
 Open     |        All    Write  Read   None
          |R W RW R W RW R W RW R W RW R W RW
 - - - - -| - - - - - - - - - - - - - - - - -
 Compat R |Y Y Y  N N N  1 N N  N N N  1 N N
        W |Y Y Y  N N N  N N N  N N N  N N N
        RW|Y Y Y  N N N  N N N  N N N  N N N
 - - - - -|
 Deny   R |C C C  N N N  N N N  N N N  N N N
 All    W |C C C  N N N  N N N  N N N  N N N
        RW|C C C  N N N  N N N  N N N  N N N
 - - - - -|
 Deny   R |2 C C  N N N  Y N N  N N N  Y N N
 Write  W |C C C  N N N  N N N  Y N N  Y N N
        RW|C C C  N N N  N N N  N N N  Y N N
 - - - - -|
 Deny   R |C C C  N N N  N Y N  N N N  N Y N
 Read   W |C C C  N N N  N N N  N Y N  N Y N
        RW|C C C  N N N  N N N  N N N  N Y N
 - - - - -|
 Deny   R |2 C C  N N N  Y Y Y  N N N  Y Y Y
 None   W |C C C  N N N  N N N  Y Y Y  Y Y Y
        RW|C C C  N N N  N N N  N N N  Y Y Y
Legend: Y = open succeeds, N = open fails with error code 05h
        C = open fails, INT 24 generated
        1 = open succeeds if file read-only, else fails with error code
        2 = open succeeds if file read-only, else fails with INT 24