Potentially dangerous function parameter in check_control()

```int check_control(int id, int key = -1);```

Parameter `key` is potentially dangerous, since the function contains a static variable that keeps the last value of key passed to it.

Currently, the parameter is responsibly only used by a call within keycontrol.cpp::process_player_keys() and keycontrol.cpp::process_set_of_keys(), both of which are called within game_frame(). There's also calls to check_control() within the lab.

The issue with this parameter is that any usage of it will have following calls use that value later. Updating the documentation to inform potential users of the danger is perhaps the minimal amount of action needed at the moment, but the usage of static variables within functions is still a bad design.