Fix race conditions in handling of unsupported drops on X

Fix race conditions in handling of unsupported drops on X

* lisp/x-dnd.el (x-dnd-handle-unsupported-drop): Adjust for new
parameters.

* src/keyboard.c (kbd_buffer, kbd_fetch_ptr, kbd_store_ptr):
Export variables.
(kbd_buffer_get_event): Ignore already handled unsupported
drops.
* src/keyboard.h: Update prototypes.

* src/termhooks.h (enum event_kind): Document meaning of
`modifiers' in UNSUPPORTED_DROP_EVENTs.

* src/xterm.c (x_dnd_send_unsupported_drop): Set event modifiers
to current level.
(x_toggle_visible_pointer): Fix fixes fallback.
(x_dnd_begin_drag_and_drop): Handle UNSUPPORTED_DROP_EVENTs
already in the keyboard buffer before starting DND.
(syms_of_xterm): Give timestamp to unsupported drop function.

* src/xterm.h: Update prototypes.