(gen_help_event): Add parameter SIZE.

(gen_help_event): Add parameter SIZE.
(kbd_buffer_events_waiting): Slightly rewritten.
(clear_event): New function.
(kbd_buffer_get_event): Use it, and clear the input_events of
HELP_EVENTs.
(init_keyboard): Remove duplicate creation of kbd_buffer_gcpro and
don't fill the newly created array with nils.
(toplevel): Convert some old-style function forward declarations
to prototypes.