Improve the mouse capture nane reported incorrectly configured

If the first capture_mouse variable is incorrect, the output is now:

- "first-bad-val" is not a valid value for variable: capture_mouse (first value).
- It might now be reset to the default value: seamless

If the second capture_mouse variable is incorrect, the output is now:

- "second-bad-val" is not a valid value for variable: capture_mouse (second value).
- It might now be reset to the default value: middlerelease

src/gui/sdlmain.cpp

@@ -2241,7 +2241,7 @@ static void GUI_StartUp(Section * sec) {
 
 	// Apply the user's mouse settings
 	Section_prop* s = section->Get_multival("capture_mouse")->GetSection();
-	const std::string control_choice = s->Get_string("mouse_control");
+	const std::string control_choice = s->Get_string("capture_mouse (first value)");
 	std::string mouse_control_msg;
 	if (control_choice == "onclick") {
 		sdl.mouse.control_choice = CaptureOnClick;
@@ -2259,7 +2259,7 @@ static void GUI_StartUp(Section * sec) {
 		assert(sdl.mouse.control_choice == CaptureOnClick);
 	}
 	std:: string middle_control_msg;
-	if (std::string(s->Get_string("middle_control")) == "middlerelease") {
+	if (std::string(s->Get_string("capture_mouse (second value)")) == "middlerelease") {
 		sdl.mouse.middle_will_release = true;
 		if (sdl.mouse.control_choice & (CaptureOnClick | CaptureOnStart))
 			middle_control_msg = " and middle-click will uncapture the mouse";
@@ -2833,8 +2833,8 @@ void Config_Add_SDL() {
 
 	// Add the mouse and middle control as sub-sections
 	Psection = Pmulti->GetSection();
-	Psection->Add_string("mouse_control", always, mouse_controls[0])->Set_values(mouse_controls);
-	Psection->Add_string("middle_control", always, middle_controls[0])->Set_values(middle_controls);
+	Psection->Add_string("capture_mouse (first value)", always, mouse_controls[0])->Set_values(mouse_controls);
+	Psection->Add_string("capture_mouse (second value)", always, middle_controls[0])->Set_values(middle_controls);
 
 	// Construct and set the help block using defaults set above
 	std::string mouse_control_help(