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.

function x = recursive_nest3 ()
  y = 5;
  f (y);
  x = y;
  g (x);
  function f (y)
    y = 10;
  endfunction

  function g (x)
    x = 10;
  endfunction
endfunction