CONCAT_DIR is fundamental in allowing software to be portable
between Unix and VMS systems. Given a directory logical name (or
environment variable) it will figure out whether to put a `/' between
the directory and the file name (for Unix systems), or to use
a `:' (for VMS systems). A restriction is that the directory must
be a logical or environment variable. Some sample calls are:
IDL> outfil = concat_dir(dir, filnam)
IDL> outfil = concat_dir('$DIR_GEN_OBS', 'obs92_23a.01')