DOS Internal - Get system file table entry for file

        Entry:
          AX = 1229h
          BX = file handle
          ???
        Return:
          ES:DI -> system file table entry for file (see INT 21/AH=52h)

        Notes:  can be called only from within DOS according to Wes Cowley,
                this actually does the same thing as INT 21/AH=3Fh