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 = varg_nest2
  [a, b] = f;
  x = a;

  if nargout == 1
    x = a;
  endif

  function [a, b] = f
    if nargout == 2
      a = b = 5;
    endif
  endfunction
endfunction