Move the definitions of all special key codes (those not passed by
ASCII value) to input/keycodes.h. Before they were spread between
osdep/keycodes.h, input/joystick.h, input/mouse.h and input/ar.h, plus
some special values in input.h. This was especially inconvenient as
the codes had to be coordinated to not conflict between the files.
The change requires a bit of ugliness as appleir.c includes
<linux/input.h> which contains various conflicting KEY_* definitions.
Work around this by adding a special preprocessor variable which can
be used to avoid defining these in keycodes.h.
Use the same mp_input_add_key_fd for all uses and add a context
argument to its callback that was before only in the event fd
callbacks. Instead of checking in input.c whether keys were inserted
to the keypress FIFO during the callback do the check in the callback
before returning and set return value accordingly.
