use new string_value method to handle value extraction errors * dirfns.cc, file-io.cc, gammainc.cc, help.cc, load-path.cc, octave-link.cc, qz.cc, regexp.cc, strfns.cc, syscalls.cc, time.cc, variables.cc: Use new string_value method.

%!test
%! global gfun
%! gfun = @fB;
%! y = fA (e);
%! assert (y, e);

%!test
%! global gfun
%! gfun = @fC;
%! y = fA (e);
%! assert (y, e);