[AD] new keyboard API in new_api_branch

[Peter Wang <tjaden@xxxxxxxxxx>]

I committed my new keyboard API patch into the new_api_branch.

* The Windows keyboard driver no longer uses pckeys.c.  It uses 
MapVirtualKey() to map hardware scancodes to ASCII characters instead 
(technique from SDL).  It still needs a rather lot of testing.

* I got rid of the `unmodchar' field in AL_KEYBOARD_EVENT.  I bet no one 
knew what this was supposed to be for, so it's not worth mentioning :-)

* The compatibility layer for the old keyboard API no longer tries to be 
clever about autorepeats.  set_keyboard_rate(0,0) can disable repeats in 
readkey().  set_keyboard_rate() with non-zero arguments enables repeats, 
but the rates are not honoured (you always get the OS repeat rate).

Peter