add joystick support

[rodan]

add generic SDL joystick support that acts as a mouse-emulator

the user does not need to select joystick/mouse input device in the main menu, the joystick init is attempted during the game start sequence.

what works:

• navigating all the menus (where the pointer max values are given by config.ini->windowRes{Width, Height}). the joystick rest location is always reset via VGA_Set_mouse().
• flying in-game (where pointer max values are expected as 640 and 480 by the game regardless of any settings I can find)

you might find the sub_main() changes a bit intrusive or plain wrong, but I need a way to tell in joystick_event_mgr() if I'm currently in flight mode or menu navigation mode.

as it's done now, if anyone is requesting VGA_Set_mouse() with exact 320,240 coords then I consider I'm in flight mode. all I can say it works great for me :)

also changed the README for an install method that does not require inno.., gog, dosemu software. also provided a list of hashes with which the user can check if the upstream files are correct.

cheers, peter

[rodan]

I will resend the push request with the joystick-related modification separate from the README change. found a bug where the yaw speed is halved after the spell menu is displayed the first time. brb.