sf_file_verify        Returns .t. if the given file_name is valid
------------------------------------------------------------------------------

    function logical sf_file_verify prototype
      parameters const char file_name,;
                 const char return_name

    Returns logical .t. if the given file_name is valid.  The return_name
    character variable will be changed to the canonicalize'd file name.

    Example:

    vardef
      char ret_buffer
    enddef
    ? sf_file_verify( "test.txt", ret_buffer )      && .t.
    ? ret_buffer                                    && C:\FORCE\TEST.TXT
    ? sf_file_verify( ",text.txt", ret_buffer )     && .f.
    ? ret_buffer                                    && *$&$%&(#(@&jHH#Uu