This routine is used to prepare a program to open a file. It first checks if
the character string referring to the file name is too long. Next it composes
the full file name (body file + extension) using the std_extention string if
the input string does not contain the extension (defined by a dot extention
delimiter). Finally it checks whether or not given file already exists. If it
does, it will rename the file to a backup file name with an extension provided
by the call. If the file does not exists, the routine will return a full file
name.
call clean_f (file_name,
file_length,std_extention,backup_extention,*err)