[gdb] Make execute_command_to_string return string on throw

The pattern for using execute_command_to_string is:
...
  std::string output;
  output = execute_fn_to_string (fn, term_out);
...

This results in a problem when using it in a try/catch:
...
  try
    {
      output = execute_fn_to_string (fn, term_out)
    }
  catch (const gdb_exception &e)
    {
      /* Use output.  */
    }
...

If an expection was thrown during execute_fn_to_string, then the output
remains unassigned, while it could be worthwhile to known what output was
generated by gdb before the expection was thrown.

Fix this by returning the string using a parameter instead:
...
  execute_fn_to_string (output, fn, term_out)
...

Also add a variant without string parameter, to support places where the
function is used while ignoring the result:
...
  execute_fn_to_string (fn, term_out)
...

Tested on x86_64-linux.

gdb/stack.c

@@ -3028,8 +3028,8 @@ frame_apply_command_count (const char *which_command,
 	       set to the selected frame.  */
 	    scoped_restore_current_thread restore_fi_current_frame;
 
-	    cmd_result = execute_command_to_string
-	      (cmd, from_tty, gdb_stdout->term_out ());
+	    execute_command_to_string
+	      (cmd_result, cmd, from_tty, gdb_stdout->term_out ());
 	  }
 	  fi = get_selected_frame (_("frame apply "
 				     "unable to get selected frame."));