Fix infloop in GC mark_kboards

Fix infloop in GC mark_kboards

* src/keyboard.c (KBD_BUFFER_SIZE): Now a constant, not a macro.
(kbd_fetch_ptr, kbd_store_ptr): These now always point somewhere
into kbd_buffer, instead of sometimes pointing just past the
end which led to serious bugs (Bug#33547).  All uses changed.
(kbd_store_ptr): No longer volatile.  This variable has not been
accessed by a signal handler for some time, it seems.
(next_kbd_event, prev_kbd_event): New functions.
(kbd_buffer_nr_stored, process_special_events): Simplify.