NAME:
     exists
 PURPOSE: (one line)
     Check for file (or directory) existence.
 DESCRIPTION:
 CATEGORY:
     File I/O
 CALLING SEQUENCE:
     flag = exists(file)
 INPUTS:
     file - string containing file name to look for.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD PARAMETERS:
 OUTPUTS:
     Return value is 1 (true) if file exists.  0 if it doesn't.
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
    93/03/29 - Written by Marc W. Buie, Lowell Observatory
    96/10/17, MWB, modified to use OPENR for Unix
    97/02/16, MWB, fixed DOS bug for dirs with trailing \
    99/06/11, MWB, Dave Osip, added clause for Macintosh computers.
    99/11/14, MWB, restructured code
    2000/10/9, Roger J. Dejus (ANL), changed code for Unix to accomodate
                 changed system response in IDL v5.3 and later.
  2004/9/21, MWB, removed obsolete call to Findfile