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 r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");