Function  3Eh   Close A File Handle  
		Close a file and release handle for reuse 
entry   AH      3Eh 
	BX      file handle 
return  flag CF 0       successful close 
		1       error 
	AX      error code if error (6) 
note 1) When executed, the file is closed, the directory is updated, and all 
	buffers for that file are flushed. If the file was changed, the time  
	and date stamps are changed to current 
     2) If called with the handle 00000, it will close STDIN (normally the  
	keyboard). 
     3) recent versions of DOS preserve AH because some versions of Multiplan
	had a bug which depended on AH being preserved