Improve quit-to-menu menu handling

This stabilizes the code that handles the menu that you land on if you
press Esc and quit to the menu.

Instead of using Game::returnmenu(), we now use the new function
Game::returntomenu() to clearly express intent that we want to return to
a specific menu. So I've added another kludge variable
Game::wasinintermission for the was-in-intermission case.

Also, I made it so that if you didn't have a main game telesave or
quicksave, you just get brought back to the main menu. Because you
shouldn't be able to go to the play menu without a quicksave or
telesave.

desktop_version/src/Input.cpp

@@ -1961,6 +1961,7 @@ void mapinput()
                 FillRect(graphics.tempBuffer, 0x000000);
                 if (game.intimetrial || game.insecretlab || game.nodeathmode) game.menukludge = true;
                 game.wasintimetrial = game.intimetrial;
+                game.wasinintermission = game.inintermission;
                 game.wasincustommode = map.custommode;
                 script.hardreset();
                 if(graphics.setflipmode) graphics.flipmode = true;